arm: add a common .lds link script

Most ARM CPUs use a very similar link script. This adds a basic
script that can be used by most CPUs.

Two new symbols are introduced which are intended to eventually be
defined on all architectures to make things easier for generic relocation
and reduce special-case code for each architecture:

__image_copy_start is the start of the text area (equivalent to the
existing _start on ARM). It marks the start of the region which must be
copied to a new location during relocation. This symbol is called
__text_start on x86 and microblaze.

__image_copy_end is the end of the region which must be copied to a new
location during relocation. It is normally equal to the start of the BSS
region, but this can vary in some cases (SPL?). Making this an explicit
symbol on its own removes any ambiguity and permits common code to always
do the right thing.

This new script makes use of CPUDIR, now defined by both Makefile and
spl/Makefile, to find the directory containing the start.o object file,
which is always placed first in the image.

To permit MMU setup prior to relocation (as used by pxa) we add an area
to the link script which contains space for this. This is taken
from commit 7f4cfcf4. CPUs can put the contents in there using their
start.S file. BTW, shouldn't that area be 16KB-aligned?

Signed-off-by: Simon Glass <sjg@chromium.org>