Silicon/Socionext: rename Emmc.c to fix build error

Silicon/Socionext/SynQuacer/Drivers/PlatformDxe/ has both an Emmc.asl
and an Emmc.c file, which the patch(edk2 commit:0a4aa20e8d446 "BaseTools:
Compile AML bytecode arrays into .obj file") both generate an Emmc.obj
in the same output directory.

To fix the build error for Developerbox platform, this patch renames
the Emmc.c.

Suggested-by: Leif Lindholm <leif@nuviainc.com>
Cc: Pierre Gondois <Pierre.Gondois@arm.com>
Signed-off-by: Masahisa Kojima <masahisa.kojima@linaro.org>
Reviewed-by: Leif Lindholm <leif@nuviainc.com>