tilegx: provide kernel support for the tilegx UART shim

The TILE-Gx chip includes an on-chip UART.  This change adds support
for using the UART from within the kernel.  The UART shim has more
functionality than is exposed here, but to keep the kernel code and
binary simpler, this is a subset of the full API designed to enable
a standard Linux tty serial driver only.

Signed-off-by: Chris Metcalf <cmetcalf@tilera.com>