review: arm32: split and enhance core_init_mmu()

Will update the commit message to:
* Splits core_init_mmu() into two functions, core_init_mmu_tables() called
  by primary CPU to create the translation tables and core_init_mmu_regs()
  called by each CPU to initialize MMU register settings.
* Adds option to map certain areas in a level 2 translation table instead
  of only level 1 mapping. Allocation of the level 2 translation tables
  is implemented in platform specific code, a weak function is provided
  in case the platform doesn't implement/need the function.

Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>