xen/arm: Add Amlogic Meson SoCs earlyprintk support

xen/arm: Add Amlogic Meson SoCs earlyprintk support

This patch adds earlyprintk support for Amlogic Meson SoC based
boards.

ATF[1] and U-boot[2] already initialize the UART for us. So no need to do it again.

Tested With:
 http://wiki.friendlyarm.com/wiki/index.php/NanoPi_K2

[1]: https://github.com/ARM-software/arm-trusted-firmware/blob/master/drivers/meson/console/aarch64/meson_console.S#L92
[2]: https://github.com/u-boot/u-boot/blob/master/drivers/serial/serial_meson.c#L44

Signed-off-by: Amit Singh Tomar <amittomer25@gmail.com>