serial: msm_geni_serial: Ensure to set clock freq correctly on RUMI

This patch adds RUMI specific changes to enable UART clock to
run at half the rate and make sure write to register gets executed
with read back of the same register.

This change is required to make console UART working on RUMI platforms.

Change-Id: I508ed1bb753e804314d179b76bbcf2c1a8e97376
Signed-off-by: Prudhvi Yarlagadda <pyarlaga@codeaurora.org>
Signed-off-by: Chandana Kishori Chiluveru <cchiluve@codeaurora.org>