mtd: nand: Fix SPL build after migration of CONFIG_SYS_NAND_SELF_INIT to Kconfig

This fixes the below build error if nand.c is included in
an SPL build.

/work/u-boot/drivers/mtd/nand/raw/nand.c: In function ‘nand_init_chip’:
/work/u-boot/drivers/mtd/nand/raw/nand.c:82:28: error: ‘nand_chip’ undeclared (first use in this function)
   82 |  struct nand_chip *nand = &nand_chip[i];
      |                            ^~~~~~~~~
/work/u-boot/drivers/mtd/nand/raw/nand.c:82:28: note: each undeclared identifier is reported only once for each function it appears in
/work/u-boot/drivers/mtd/nand/raw/nand.c:84:20: error: ‘base_address’ undeclared (first use in this function); did you mean ‘base_addr’?
   84 |  ulong base_addr = base_address[i];
      |                    ^~~~~~~~~~~~
      |                    base_addr

Fixes: 068c41f1 ("Finish conversion CONFIG_SYS_NAND_SELF_INIT to Kconfig")
Signed-off-by: Roger Quadros <rogerq@kernel.org>
Reviewed-by: Michael Trimarchi <michael@amarulasolutions.com>
Signed-off-by: Dario Binacchi <dario.binacchi@amarulasolutions.com>