Tegra: allow platforms to override plat_core_pos_by_mpidr()

This patch makes the default implementation of plat_core_pos_by_mpidr()
as weakly linked, so that platforms can override it with their own.

Tegra186, for one, does not have CPU IDs 2 and 3, so it has its own
implementation of plat_core_pos_by_mpidr().

Change-Id: I7a5319869c01ede3775386cb95af1431792f74b3
Signed-off-by: Varun Wadekar <vwadekar@nvidia.com>