Replace magic numbers in linkerscripts by PAGE_SIZE

When defining different sections in linker scripts it is needed to align
them to multiples of the page size. In most linker scripts this is done
by aligning to the hardcoded value 4096 instead of PAGE_SIZE.

This may be confusing when taking a look at all the codebase, as 4096
is used in some parts that aren't meant to be a multiple of the page
size.

Change-Id: I36c6f461c7782437a58d13d37ec8b822a1663ec1
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>