SbsaQemu: AcpiDxe: Read MPIDR from device tree

The Qemu device tree for Sbsa platform now contains MPIDR value
for every CPU in the form of "reg" property under every CPU's
node. Hence, add a function that provides support to read this
value from the device tree.

Signed-off-by: Tanmay Jagdale <tanmay.jagdale@linaro.org>
Tested-by: Graeme Gregory <graeme@nuviainc.com>
Reviewed-by: Leif Lindholm <leif@nuviainc.com>