feat(imx8mp): add BL31 PIE support

Enable PIE support so the BL31 firmware can be loaded from anywhere
within the OCRAM (SRAM). How important this is shows the back and forth
of the BL31_BASE address starting with TF-A v2.5. Since then the
BL31_BASE address wasn't stable and choosing the correct combination of
SPL version loadaddress and TF-A version loadaddr was tricky.

For the PIE support we only need to replace the BL31_BASE by the
BL31_START which is a relocatable symbol and to enable it by setting
ENABLE_PIE := 1.

Signed-off-by: Marco Felsch <m.felsch@pengutronix.de>
Change-Id: I4214db1f27120f9f9cd1413ccd7a5a7d095ff45d