msm_serial_hs: Use RFR GPIO to communicate remote uart

CTS and RFR GPIOs are used for hardware flow control between
two uarts. RFR GPIO provides current status of uart receiver
to remote uart and based on this remote uart sends data.
Hence use this functionality to prevent remote uart to send
data when port is being opened, baud rate change operation
is in progress and port is being closed.

This change also set UART_DM_MR2 (Bit: 8 and 9) before
configuring connected UART GPIOs to prevent RX_BREAK character
and any character having PARTIY or FRAMING error which can be
generated due to configuration of those GPIOs. Allows those
character once port is opened successfuly.

This change also zeroed uart rx software buffer which would
help to see how many bytes copied by ADM from received data
with UART RX FIFO to this memory. This would be useful if
there is any Rx Stall issue is seen.

CRs-Fixed: 495564
Change-Id: I69382994356073c4c86f205015503c935ad09e8d
Signed-off-by: Mayank Rana <mrana@codeaurora.org>