ARM: realview: add an DT SMP boot method

ARM: realview: add an DT SMP boot method

This adds an SMP boot method for the ARM RealView reference
designs. We also select HAVE_SMP by default and make it use
SMP_ON_UP so we only need to support one single kernel across
the RealView reference designs when using DT.

The RealViews need to have the SCU (Snoop Control Unit)
activated on boot, and this is now done by looking up its
address from the device tree and initializing it and counting
the available cores.

The RealViews boot by using a magic address register in the
system controller (SYS_FLAGS) to store the boot address,
the ROM will then read this register to the PC when the CPUs
are taken out of WFI. This code uses a handle to the syscon
regmap to access this register.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>