serial: mps2-uart: add support for early console

This adds support early console for MPS2 UART which can be enabled via
earlycon=mps2,0x40004000

Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Vladimir Murzin <vladimir.murzin@arm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>