defconfig: msm: Decide UART sampling rate based on config

As earlycon can't have QUP HW version awareness, decision
for UART sampling rate is taken based on the configuration.

Change-Id: I2a8674752bcd360e47a90c3c718d4bede7af0dc4
Signed-off-by: Mitul Golani <mgolani@codeaurora.org>