uapi: wcd-spi-ac: add interface for client driver

The wcd-spi-ac (access control) driver exposes character
device to userspace to indicate information regarding
WCD SPI usage. Add uapi header file with the commands
and data expected by the character driver.

Change-Id: I41dfc3181041c7e7a1bad8695fa723d3a965036b
Signed-off-by: Bhalchandra Gajare <gajare@codeaurora.org>