product/rdn2: add helper function to update reset syndrome in sds

The SCP PIK address space (0x5000.0000 to 0x5FFF.FFFF) should be read
with 32-bit aligned addresses. But, SDS module performs reads at
byte-aligned address. So this helper function is used to first read the
Reset Syndrome value from the RESET_SYNDROME register and update the
reset syndrome value in the shared data storage using SDS write API.

Change-Id: I2b298f11c087d28b274f4003e158025c49ec194e
Signed-off-by: Thomas Abraham <thomas.abraham@arm.com>