ARM: u300: switch to using syscon regmap for board

This switches the code using a local remapping of the
system controller to enable the U300 board to be
self-powered over to making the U300-specific syscon
compatible with the MFD generic syscon driver, selecting
the generic syscon driver, and augmenting the board
power code to pick the regmap and manipulate the syscon
from the regmap side of things.

Cc: Dong Aisheng <dong.aisheng@linaro.org>
Suggested-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>