add addrespace definition for sh2a.

Newfile: arch/sh/include/cpu-sh2a/cpu/addrspace.h

This file seems had be removed to use fallback (cpu-common/cpu/addrspace.h),
but, I'd like to add sh2a specific file here, because
1. the values defined there are not suitable for sh2a.
2. I don't think there is "common" definition for these values.

Values are chosen by consideration of followings...
 P1 is 0. perhaps no question.
 P2 is from hardware manual, which says no-cache area starts at 20000000.
 It means that P? space size=20000000.
 P3 is P2+size since asm/ptrace.h uses P3 as a end of P2.
 P4 is P3+size since asm/fixup.h uses P4 as a end of P3.

Signed-off-by: Takashi YOSHII <yoshii.takashi@renesas.com>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>