memory: Kconfig: Drop dependency on MACH_JZ4780 for jz4780

Depending on MACH_JZ4780 prevent us from creating a generic kernel that
works on more than one MIPS board. Instead, we just depend on MIPS being
set.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Reviewed-by: Boris Brezillon <bbrezillon@kernel.org>
Signed-off-by: Paul Burton <paul.burton@mips.com>