gdsc: Move GDSC driver to driver/clk/qcom

Architectural changes in the ARM Linux kernel tree mandate
the eventual removal of the mach-* directories. Move GDSC driver
to drivers/clk/qcom since it's closely related to clocks.

Change-Id: I3c920cf235c5493b1d50661e0c1db78c3c721336
Signed-off-by: Junjie Wu <junjiew@codeaurora.org>