msm: make debugging UART (for DEBUG_LL) configurable

Provides options to select one of the three "lowspeed" UARTs
on MSM7k SoCs for DEBUG_LL output from the zImage decompressor
and kernel.

Signed-off-by: Brian Swetland <swetland@google.com>
Signed-off-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Daniel Walker <dwalker@codeaurora.org>