spl: add fixed memory node in target fdt also when loading ATF

In a loading chain SPL -> ATF (->OP-TEE) -> U-Boot, ATF and a subsequent
OP-TEE will re-use the same fdt as the U-Boot target and may need the
information about usable memory ranges.

Especially OP-TEE needs this to initialize dynamic shared memory
(the only type U-Boot implements when talking to OP-TEE).

So allow spl_fixup_fdt() to take a fdt_blob argument, falling back to
the existing CONFIG_SYS_SPL_ARGS_ADDR if needed and call it from the
ATF path as well.

Signed-off-by: Heiko Stuebner <heiko.stuebner@theobroma-systems.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>