spi: aspeed: SPI dirmap read support

From the HW point of view, the performance of
command read mode is greater than user mode slightly.
Thus, dirmap read framework is introduced to achieve
this goal. In dirmap_create, command read mode is
configured. Usually, the decoded address area with flash
size is assigned to each CS. CPU can thus access the
SPI flash as normal memory in dirmap_read function.

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