Platform: Specify the starting address of privileged secure data area

In multi-core topology, Armv6-M/Armv7-M MPU requires a MPU region
base address to align with its size. Specify the starting address
of privileged secure data area when the multi-core platform defines
the S_DATA_PRIV_START. It can make MPU configuration more easier to
satisfy Armv6-M/Armv7-M MPU alignment on multi-core platforms.

Change-Id: I57904b772b352b1bbd1b4b9015093da258358e00
Signed-off-by: David Hu <david.hu@arm.com>