ARM: vexpress: refine dependencies for new code

The versatile express changes for 3.16 introduced a number of
build regressions for randconfig kernels by not tracking dependencies
between the components right.

This patch tries to rectify that:

* the mach-vexpress code cannot link without the syscfg driver,
  which in turn needs MFD_VEXPRESS_SYSREG
* various drivers call devm_regmap_init_vexpress_config(), which
  has to be exported so it can be used by loadable modules
* the configuration bus uses OF DT helper functions that are not
  available to platforms disable CONFIG_OF
* The sysreg driver exports GPIOs through gpiolib, which can
  be disabled on some platforms.
* The clocksource code cannot be built on platforms that don't
  use modern timekeeping but rely on gettimeoffset.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>