ARM: keystone: Switch over to coherent memory address space

With late code patching updates for LPAE machines has merged now and
memblock conversion from bootmem is on its way, Keystone can switch to
the coherent memory address space which starts beyond 4GB boundary.
The idmap alias needs are managed via virt_to_idmap() for boot purpose.

Tested-by: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: Santosh Shilimkar <santosh.shilimkar@ti.com>