msm: allow uart to be conditionally disabled

Some MSM targets don't select the debug UART in this way. For those we
need to disable this selection mechanism.

Signed-off-by: Daniel Walker <dwalker@codeaurora.org>
Signed-off-by: Jeff Ohlstein <johlstei@codeaurora.org>
Signed-off-by: Daniel Walker <dwalker@codeaurora.org>