soc: ambiq: add common cache handling for apollo5x soc

The buf_in_nocache function is to be used by various device drivers
to check if buffer is in noncacheable region.
The cacheable DMA buffer shall be put into section .ambiq_dma_buff
due to certain restrictions of the SoC.

Signed-off-by: Swift Tian <swift.tian@ambiq.com>