serial: msm-geni: don't rely on parent misc device

commit 1b15483d ("misc: add Qualcomm GENI SE QUP device driver")
introduced support for platform-specific oversampling values, necessary
to configure the UART clocks on all platforms at runtime. However it
relies in probing a parent device. Despite the DM_FLAG_PRE_RELOC flag,
this is not done consistently during boot.

Instead, take another approach by relying on ofnode_ helpers to read the
serial engine base address and do the read directly. This fixes early
UART on boards with a non-default oversampling rate.

Signed-off-by: Caleb Connolly <caleb.connolly@linaro.org>