sh: Only provide a PCLK definition for legacy CPG CPUs.

As CPUs are migrated over to more fully-featured clock frameworks of
their own and off of the legacy CPG code, they no longer have any real
need for defining the PCLK value. The PCLK define in itself is already
fairly misleading, as many boards get their input clocks from different
sources, making this value fairly arbitrary anyways.

Outside of the legacy CPG clock framework, the only place where this
value is used is for deriving CLOCK_TICK_RATE, which we set back to the
legacy PIT value that it was before the PCLK definitions were added in
the first place.

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