mfd: Versatile Express system registers driver

mfd: Versatile Express system registers driver

This is a platform driver for Versatile Express' "system
register" block. It's a random collection of registers providing
the following functionality:

- low level platform functions like board ID access; in order to
  use those, the driver must be initialized early, either statically
  or based on the DT

- config bus bridge via "system control" interface; as the response
  from the controller does not generate interrupt (yet), the status
  register is periodically polled using a timer

- pseudo GPIO lines providing MMC card status and Flash WP#
  signal control

- LED interface for a set of 8 LEDs on the motherboard, with
  "heartbeat", "mmc0" and "cpu0" to "cpu5" as default triggers

Signed-off-by: Pawel Moll <pawel.moll@arm.com>