powerpc/fsl-booke: Move the entry setup code into a seperate file

This patch only moves the initial entry code which setups the mapping
from what ever to KERNELBASE into a seperate file. No code change has
been made here.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>