OMAP7XX: Serial: Remove duplicate omap850 code

This patch is part of a series which unifies all duplicated code between
omap730 and omap850. All cpu checks are converted to cpu_is_omap7xx() and
CONFIG_ARCH_OMAP850 is added to all CONFIG_ARCH_OMAP730 checks.

Signed-off-by: Alistair Buxton <a.j.buxton@gmail.com>
Reviewed-by: Zebediah C. McClure <zmc@lurian.net>