sh: clkfwk: Wire up clk_get_sys() support.

This stubs in clk_get_sys() from the ARM clkdev implementation.
Tentatively conver the clk_get() lookup code to use this, and once the
rest of the in-tree users are happy with this, it can replace the
fallback lookups.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>