sh: Fixup R7780RP iVDR clock enable.

The iVDR clock enable bit happens to actually reside in a rather
different place than what is documented, so fix it up accordingly.
This fixes up SATA boot for some of the R7780RP boards that didn't
default-enable the clock in the loader.

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