sh: Kill off some superfluous legacy PMB special casing.

The __va()/__pa() offsets and the boot memory offsets are consistent for
all PMB users, so there is no need to special case these for legacy PMB.
Kill the special casing off and depend on CONFIG_PMB across the board.
This also fixes up yet another addressing bug for sh64.

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