serial: Make SERIAL_MVEBU_UART depend on ARCH_MVEBU

Enabling support for the UART on Marvell EBU SoCs only make sense when
compiling for Marvell EBU SoCs, unless compile-testing.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>