In some cases low level drivers might want to update the
SPI transfer clock (e.g. during firmware download).

This patch adds this support. Without any modification the
driver will use the default SPI clock (from pdata or device tree).
Signed-off-by: Vincent Cuissard <cuissard@marvell.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>