ARM: realview: imply device tree boot

This reduces the Kconfig for the RealView by assuming we are
always booting from the device tree, and removing all the uses
of CONFIG_REALVIEW_DT and replacing with CONFIG_ARCH_REALVIEW.

Further:

- Drop REALVIEW_HIGH_PHYS_OFFSET: we don't use this with device
  tree.

- Drop the REALVIEW_EB_ARM11MP_REVB option: we now handle this
  by simply using another device tree.

- Drop the PB1176 secure flash option: this is defined in the
  PB1176 device tree but marked as "disabled", so users who
  want to use it can simply enable it in the device tree and
  go hacking around.

Cc: Brian Norris <computersforpeace@gmail.com>
Cc: Marc Zyngier <marc.zyngier@arm.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>