sh: VoyagerGX cleanups and 8250 UART support.

This adds the VoyagerGX UART to the RTS7751R2D setup
code, and cleans up a few build issues.

Signed-off-by: Manuel Lauss <mano@roarinelk.homelinux.net>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>