mtd: spi-nor: aspeed: set the decoding size to at least 2MB for AST2600

In AST2600, the unit of SPI CEx decoding range register is 1MB, and end
address offset is set to the acctual offset - 1MB. If the flash only has
1MB, the end address will has same value as start address, which will
causing unexpected errors.

This patch set the decoding size to at least 2MB to avoid decoding errors.

Tested:
root@bletchley:~# dmesg | grep "aspeed-smc 1e631000.spi: CE0 window"
[   59.328134] aspeed-smc 1e631000.spi: CE0 window resized to 2MB (AST2600 Decoding)
[   59.343001] aspeed-smc 1e631000.spi: CE0 window [ 0x50000000 - 0x50200000 ] 2MB
root@bletchley:~# devmem 0x1e631030
0x00100000

Tested-by: Jae Hyun Yoo <quic_jaehyoo@quicinc.com>
Signed-off-by: Potin Lai <potin.lai@quantatw.com>
[ clg : Ported on new spi-mem driver ]
Signed-off-by: Cédric Le Goater <clg@kaod.org>
Link: https://lore.kernel.org/r/20220509175616.1089346-12-clg@kaod.org
Signed-off-by: Mark Brown <broonie@kernel.org>