armv8: Add read_mpidr() function

This patch adds the read_mpidr() function which returns the
MPIDR_EL1 register value

Signed-off-by: Sergey Temerkhanov <s.temerkhanov@gmail.com>
Signed-off-by: Radha Mohan Chintakuntla <rchintakuntla@cavium.com>

Reviewed-by: Simon Glass <sjg@chromium.org>