clocks: qcom: add clocks for SDM845 debug uart

Allows to change clock frequency of debug uart,
thus supporting wide range of baudrates.
Enable / disable functionality is not implemented yet.
In most use cases of SDM845 (i.e. mobile phones and tablets)
it's not needed, because qualcomm first stage bootloader leaves it
initialized, and on the other hand there's no possibility to
replace signed first stage bootloader with u-boot.

Signed-off-by: Dzmitry Sankouski <dsankouski@gmail.com>
Cc: Ramon Fried <rfried.dev@gmail.com>
Cc: Tom Rini <trini@konsulko.com>