arm64: Allow boot on cpus other than logical cpu 0

Kernel assumes boot cpu to be always on logical cpu 0.
At times there are benefits to boot kernel on other cpus
as well. On a heterogeneous multiprocessor system, booting
on a high-performance core can improve the boot time.

This patch removes the assumption that boot-cpu is logical
cpu 0. Device tree is looked up very early in boot to
determine the boot-cpus logical id. It also fixes hard-coding
of logical cpu 0 done at various places.

Change-Id: I27f1b2762db23a0af84fa5289f4b68ed31d84560
Signed-off-by: Yadu MG <ymg@codeaurora.org>