serial: Make SERIAL_MPS2_UART depend on ARCH_MPS2

Enabling support for the UART on the ARM MPS2 platform only make sense
when compiling for the ARM MPS2 platform, unless compile-testing.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Vladimir Murzin <vladimir.murzin@arm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>