Translate embedded assembly into IO writes which is more
readable.

Also hardcode cm_base at boot time instead of reading from CP0.

Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20210127065424.114125-5-jiaxun.yang@flygoat.com>
[PMD: Kept code comments]
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>