MIPS: Alchemy: add helpers to access static memory ctrl registers.

This patch changes the static memory controller registers to offsets
from base, prefixes them with AU1000_ to avoid silent failures due to
changed addresses and introduces helpers to access them.

No functional changes, comparing assembly of a few select functions shows
no differences.

Signed-off-by: Manuel Lauss <manuel.lauss@gmail.com>
Cc: Linux-MIPS <linux-mips@linux-mips.org>
Patchwork: https://patchwork.linux-mips.org/patch/7463/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>