slatecom_interface: add slatecom_interface driver

slatecom_interface driver is use for communication from
slate PIL and slatecom spi to userspace.
Changes are ported from bgcom_interface driver.

Change-Id: I92379ca3d691a17f8e6507f1135f2ab340a9a8ae
Signed-off-by: Ajit Kumar <kajit@codeaurora.org>