ARM: mmp: rename SHEEVAD to GPLUGD

GuruPlugD was initially named to be SHEEVAD, and it's causing naming
confusion in the mach-types database. Make it consistent by renaming
to GPLUGD.

Reported-and-Acked-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Tanmay Upadhyay <tanmay.upadhyay@einfochips.com>
Signed-off-by: Eric Miao <eric.y.miao@gmail.com>