drivers: cma: Fix wrong CMA selected region size default value

Kconfig lists CMA_SIZE_SEL_ABSOLUTE as the default value fo the CMA
selected region size, but that option isn't available in the defined
choices. Set the default to CMA_SIZE_SEL_MBYTES instead.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>