Support spi driver and can detect MX25U1635E flash on AE3XX board.

Verification:

sf probe 0:0 50000000 0
spi_flash_std_probe(sf_Probr.c)
	spi_flash_probe_slave(sf_Probr.c)
	SF: Detected mx25u1635e with page size 256 Bytes, erase size 4 KiB, total 2 MiB
	NDS32 # sf test 0x100000 0x1000
	SPI flash test:
	0 erase: 34 ticks, 117 KiB/s 0.936 Mbps
	1 check: 15 ticks, 266 KiB/s 2.128 Mbps
	2 write: 21 ticks, 190 KiB/s 1.520 Mbps
	3 read: 11 ticks, 363 KiB/s 2.904 Mbps
	Test passed
	0 erase: 34 ticks, 117 KiB/s 0.936 Mbps
	1 check: 15 ticks, 266 KiB/s 2.128 Mbps
	2 write: 21 ticks, 190 KiB/s 1.520 Mbps
	3 read: 11 ticks, 363 KiB/s 2.904 Mbps

Signed-off-by: rick <rick@andestech.com>