The 'plat_core_pos_by_mpidr' handler gets called very early during boot
and the compiler generated code overwrites the caller's registers.

This patch converts the 'plat_core_pos_by_mpidr' handler into an assembly
function and uses registers x0-x3, to fix this anomaly.

Change-Id: I8d974e007a0bad039defaf77b11a180d899ead3c
Signed-off-by: Varun Wadekar <vwadekar@nvidia.com>