sh: clkfwk: Make clock-cpg usable for non-legacy platforms.

This adds a new SH_CLK_CPG for parts that have CPG support.
SH_CLK_CPG_LEGACY is made to depend on this, and still needs to be set
for platforms that want clock-cpg to register the legacy clocks. With
this new config item in place, it is now possible to start layering more
generic CPG code in place while other platforms transition off of the
legacy clocks.

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