mtd: nand: mxc_nand: Fix is_16bit_nand()

Currently is_16bit_nand() is a per SoC function and it decides the bus nand
width by reading some boot related registers.

This method works when NAND is the boot medium, but does not work if another
boot medium is used. For example: booting from a SD card and then using NAND
to store the environment variables, would lead to the following error:

NAND bus width 16 instead 8 bit
No NAND device found!!!
0 MiB

Use CONFIG_SYS_NAND_BUSWIDTH_16BIT symbol to decide the bus width.

If it is defined in the board file, then consider 16-bit NAND bus-width,
otherwise assume 8-bit NAND is used.

This also aligns with Documentation/devicetree/bindings/mtd/nand.txt, which
states:

nand-bus-width : 8 or 16 bus width if not present 8

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Scott Wood <scottwood@freescale.com>
Reviewed-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>

README

@@ -3759,8 +3759,9 @@ Low Level (hardware related) configuration options:
 		Defined to tell the NAND controller that the NAND chip is using
 		a 16 bit bus.
 		Not all NAND drivers use this symbol.
-		Example of driver that uses it:
+		Example of drivers that use it:
 		- drivers/mtd/nand/ndfc.c
+		- drivers/mtd/nand/mxc_nand.c
 
 - CONFIG_SYS_NDFC_EBC0_CFG
 		Sets the EBC0_CFG register for the NDFC. If not defined

drivers/mtd/nand/mxc_nand.c

@@ -98,45 +98,14 @@ static struct nand_ecclayout nand_hw_eccoob2k = {
 #endif
 #endif
 
-#ifdef CONFIG_MX27
 static int is_16bit_nand(void)
 {
-	struct system_control_regs *sc_regs =
-		(struct system_control_regs *)IMX_SYSTEM_CTL_BASE;
-
-	if (readl(&sc_regs->fmcr) & NF_16BIT_SEL)
-		return 1;
-	else
-		return 0;
-}
-#elif defined(CONFIG_MX31)
-static int is_16bit_nand(void)
-{
-	struct clock_control_regs *sc_regs =
-		(struct clock_control_regs *)CCM_BASE;
-
-	if (readl(&sc_regs->rcsr) & CCM_RCSR_NF16B)
-		return 1;
-	else
-		return 0;
-}
-#elif defined(CONFIG_MX25) || defined(CONFIG_MX35)
-static int is_16bit_nand(void)
-{
-	struct ccm_regs *ccm = (struct ccm_regs *)IMX_CCM_BASE;
-
-	if (readl(&ccm->rcsr) & CCM_RCSR_NF_16BIT_SEL)
-		return 1;
-	else
-		return 0;
-}
+#if defined(CONFIG_SYS_NAND_BUSWIDTH_16BIT)
+	return 1;
 #else
-#warning "8/16 bit NAND autodetection not supported"
-static int is_16bit_nand(void)
-{
 	return 0;
-}
 #endif
+}
 
 static uint32_t *mxc_nand_memcpy32(uint32_t *dest, uint32_t *source, size_t size)
 {