MIPS: Alchemy: introduce helpers to access SYS register block.

This patch changes all absolute SYS_XY registers to offsets from the
SYS block base, prefixes them with AU1000 to avoid silent failures due
to changed addresses, and introduces helper functions to read/write
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/7464/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>