clk: change return type of clk_get_parent_rate from long long to ulong

All functions getting and setting clock rate use ulong for rate, only
clk_get_parent_rate is an exception. Change the return value to match
other clock rate funcrions.

Most users directly assign the rate to unsigned long anyway, and the few
users that use u64 (not s64) multiply the rate so they may need the
extra bits for the result in their use case.

Fixes: 4aa78300 ("dm: clk: Define clk_get_parent_rate() for clk operations")
Signed-off-by: Michal Suchanek <msuchanek@suse.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Sean Anderson <seanga2@gmail.com>
Link: https://lore.kernel.org/r/20220928103757.11870-1-msuchanek@suse.de