sh: Setup boot CPU VBR early to enable early page faults.

vmemmap and the vmsplit code amongst others need to be able to take page
faults much earlier than trap_init() time, so move this in to the early
CPU initialization. VBR setup for secondary CPUs is already handled
through start_secondary(), so we only need to do this for the boot CPU.

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