mfd: vexpress: Define the device as MFD cells

This patch - finally, after over 6 months! :-( - addresses
Samuel's request to split the vexpress-sysreg driver into
smaller portions and define the device in a form of MFD
cells:

* LEDs code has been completely removed and replaced with
  "gpio-leds" nodes in the tree (referencing dedicated
  GPIO subnodes in sysreg - bindings documentation updated);
  this also better fits the reality as some variants of the
  motherboard don't have all the LEDs populated

* syscfg bridge code has been extracted into a separate
  driver (placed in drivers/misc for no better place)

* all the ID & MISC registers are defined as sysconf
  making them available for other drivers should they need
  to use them (and also to the user via /sys/kernel/debug/regmap
  which can be helpful in platform debugging)

Signed-off-by: Pawel Moll <pawel.moll@arm.com>
Acked-by: Lee Jones <lee.jones@linaro.org>