MAINTAINERS: Add entry for Qualcomm clock drivers

Most SoC specific clock drivers are picked by respective SoC maintainer
and then sent to the clock maintainers on their way upstream.

This has however not been the case for the Qualcomm clock drivers -
which doesn't actually have a maintainer per MAINTAINERS and where the
framework maintainers have just carried the Qualcomm effort as well,
presumably as a result of Stephen's history.

Move the maintainership of the Qualcomm clock drivers to use the same
model as other SoC vendors and document the ownership by actually
introducing an entry in MAINTAINERS.

Cc: Stephen Boyd <sboyd@kernel.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Acked-by: Stephen Boyd <sboyd@kernel.org>
Link: https://lore.kernel.org/r/20211203013901.3460496-1-bjorn.andersson@linaro.org