n1sdp: restructure the function to handle the copy of AP Firmware

The mpu configuration is also updated to reflect the correct
SRAM size. This enables the functions to copy the AP firmware
to AP SRAM.

Change-Id: I27b11a4eeb078b15fac4e09ab33b225a2c87659c
Signed-off-by: Deepak Pandey <Deepak.Pandey@arm.com>