msm_serial_hs_lite: Save UART registers if stuck

Preserve and attempt to print out relevant UARTDM core
registers, followed by panicing the system, if the
polling-mode serial console fails to print a character as a
result of the TX buffer remaining full for more than 100
character transmit times. This information is useful for
troubleshooting extremely rare cases where the serial
console unexpectedly stops producing output.

Change-Id: Iaaf13a01cf6ce33637a45f33b9ae6fce939cf7cd
Signed-off-by: Stepan Moskovchenko <stepanm@codeaurora.org>