ARM: at91: Get the Chip ID of SAMA5D2 SiP

The SAMA5D2 SiP(System in Package) has different Chip IDs in the
CHIPID and CHIP_EXID registers.

Signed-off-by: Wenyou Yang <wenyou.yang@microchip.com>
Reviewed-by: Simon Glass <sjg@chromium.org>