ARM: ep93xx: add regmap aux_dev

ARM: ep93xx: add regmap aux_dev

The following driver's should be instantiated by ep93xx syscon driver:

- reboot
- pinctrl
- clock

They all require access to DEVCFG register with a shared lock held, to
avoid conflict writing to swlocked parts of DEVCFG.

Provide common resources such as base, regmap and spinlock via auxiliary
bus framework.

Signed-off-by: Nikita Shubin <nikita.shubin@maquefel.me>
Tested-by: Alexander Sverdlin <alexander.sverdlin@gmail.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Stephen Boyd <sboyd@kernel.org>