memory: omap-gpmc: remove unneeded asm/mach-types.h inclusion

The driver does not use macros from asm/mach-types.h (neither MACH_TYPE
nor machine_is_xxx()).  Removal of this include allows compile testing
on non-ARM architectures which lack this header.

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Link: https://lore.kernel.org/r/20200724074038.5597-2-krzk@kernel.org