spi: aspeed: Adjust decoded range size support

There are some known HW problems about decoded
range register configurations on existing AST2500 and
AST2600 platforms. Additional callback function,
adjust_decoded_sz, is added to solve these problems
on each platform. Besides, aspeed_spi_trim_decoded_size
function is added to modify overall decoded address
size for fitting the maximum AHB decoded size.

Signed-off-by: Chin-Ting Kuo <chin-ting_kuo@aspeedtech.com>