spl: allow a separate TEXT_BASE, LDSCRIPT and MAX_SIZE for TPL

spl: allow a separate TEXT_BASE, LDSCRIPT and MAX_SIZE for TPL

For the bringup of the RK3368, we need to support TPL and SPL running
from different addresses... which requires both stages to use a
distinct TEXT_BASE.

This commit adds support for having a separate LDSCRIPT for TPL (which
is expected to make use of the TPL_MAX_SIZE define) and for having a
the option of defining TPL_TEXT_BASE and having the TPL stage linked
against this address.

Note that the handling of the TEXT_BASE is designed to not interfere
with the previous assumption that SPL_TEXT_BASE should be used for TPL
as well, unless TPL_TEXT_BASE is defined.  For this reason, the test
in Makefile.spl uses the following (seemingly redundant checks):
 1. looks for $(SPL_TPL_)TEXT_BASE
 2. looks for SPL_TEXT_BASE (even when building in TPL)

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Simon Glass <sjg@chromium.org>