doc: fix: Replace SPL_OF_PLATDATA with OF_PLATDATA in examples

The of-plat.rst file till this change has been using

This is at best misleading as SPL_OF_PLATDATA is always defined when we
want to use this SPL tinification feature (also in U-Boot proper).
As a result the OF_PLATDATA SPL specific code is also compiled in when
U-Boot proper is build.

Signed-off-by: Lukasz Majewski <lukma@denx.de>
Reviewed-by: Peng Fan <peng.fan@nxp.com>
Reviewed-by: Simon Glass <sjg@chromium.org>