memory: omap-gpmc: Include <linux/sizes.h> for SZ_16M

The driver uses SZ_16M which is defined in include/linux/sizes.h.  On
ARM it was pulled by other headers but its inclusion is necessary for
compile testing on other architectures.

This fixes build error when compile testing on i386 architecture:

    drivers/memory/omap-gpmc.c: In function ‘gpmc_cs_remap’:
    drivers/memory/omap-gpmc.c:961:12: error: ‘SZ_16M’ undeclared (first use in this function)

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>