sh: Fix the offset from P1SEG/P2SEG where we map RAM

We need to map the gap between 0x00000000 and __MEMORY_START in the PMB,
as well as RAM.

With this change my 7785LCR board can switch to 32bit MMU mode at
runtime.

Signed-off-by: Matt Fleming <matt@console-pimps.org>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>