serial: msm-geni: remove invalid se-clk clock name

There is only one clock supplier to the serial IP, thus getting it by
name is not needed, also note that "clock-names" property is not listed
under doc/device-tree-bindings/serial/msm-geni-serial.txt, and finally
"se-clk" clock name is invalid, if added, it shall get "se" value like
it's already described in Linux device tree documentation.

Signed-off-by: Vladimir Zapolskiy <vladimir.zapolskiy@linaro.org>
Reviewed-by: Konrad Dybcio <konrad.dybcio@linaro.org>