sh: Rework P2 to only include kernel text.

This effectively neutralizes P2 by getting rid of P1 identity mapping
for all available memory and instead only establishes a single unbuffered
PMB entry (16MB -- the smallest available) that covers the kernel.

As using segmentation for abusing caching attributes in drivers is no
longer supported (and there are no drivers that can be enabled in 32-bit
mode that do this), this provides us with all of the uncached access
needs by the kernel itself.

Drivers and their ilk need to specify their caching attributes when
remapping through page tables, as usual.

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