drivers/bus/spi: Add support for Intel GSPI controller

BUG=b:35583330
BRANCH=None
TEST=Compiles successfully

Change-Id: I65889e551a6c949e0ebc8dd8fa930986276f3e60
Signed-off-by: Furquan Shaikh <furquan@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/474109
Reviewed-by: Duncan Laurie <dlaurie@chromium.org>
4 files changed