docs: Update note about plat/arm in Porting Guide

Platforms are not allowed to use any file inside include/plat/arm or
plat/arm to prevent dependencies between Arm platforms and non-Arm
platforms.

Change-Id: I6dc336ab71134c8d2758761fac0e4716e2d7e6ff
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>