doc: Update arg usage for BL2 and BL31 setup functions

Updated the porting guide for the usage of received arguments
in BL2 and BL32 setup functions in case of Arm platform.

Signed-off-by: Manish V Badarkhe <Manish.Badarkhe@arm.com>
Change-Id: Ia83a5607fed999819d25e49322b3bfb5db9425c0
1 file changed