drivers: spi: atcspi200: determining if spi is used as a flash controller

XIP may indicate that the program is executed either in local memory
or flash. However, the SPI node is only used as a flash controller
when the program is executed in flash.
Therefore, optimize the related checks to ensure that when XIP is
enabled but the program is executed in local memory, the SPI node
can still be used.

Signed-off-by: Kevin Wang <kevinwang821020@google.com>