Move platform_helpers.S to each platform's folder

In practice, all the functions in this file are platform-specific. It is
better to force all platforms to implement than having some sort of weak
function placeholder.

Porting guide updated.

Change-Id: I5beeeb10bec6fe5178b24503d6da8ca66074a8c6
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>