drivers: soc: qcom: Add slatecom driver

slatecom driver provides a set of apis to perform
register, fifo, ahb read and write operations with
slate coprocessor over spi interface.
slatecom driver is ported from bgcom driver.

Change-Id: Ibaacf4748d893511196e4a66ed17ff26a854809b
Signed-off-by: Ajit Kumar <kajit@codeaurora.org>