sh: Fix up SH7710 VoIP-GW build.

The only board-specific bits that existed here were for setting up the
IRQs, which are now handled by the SH7710 CPU support code instead. As
there's nothing else to do for setup, kill off the board support code
and have the defconfig use the generic machvec instead.

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