Move console drivers to AArch64 folder ...

This patch moves the various assembly console drivers
into `aarch64` architecture specific folder. Stub files,
which include files from new location, are retained at the
original location for platform compatibility reasons.

Change-Id: I0069b6c1c0489ca47f5204d4e26e3bc3def533a8

Driver for 16550 UART interface ...

This patch adds driver for the 16550 UART interface. The driver is exposed
as a console, which platforms can use to dump their boot/crash logs.

Signed-off-by: Varun Wadekar <vwadekar@nvidia.com>