SPI: OMAP: remove unnecessary includes of plat/clock.h

Remove unnecessary includes of plat/clock.h from the OMAP SPI
controller drivers.  These need to be removed to build multi-subarch
ARM kernels which include these drivers.

Signed-off-by: Paul Walmsley <paul@pwsan.com>
Cc: Grant Likely <grant.likely@secretlab.ca>
Acked-by: Grant Likely <grant.likely@secretlab.ca>