feat(imx8mn): add BL31 PIE support

Enable PIE support so the BL31 firmware can be loaded from anywhere
within the OCRAM (SRAM). For the PIE support we only need to replace the
BL31_BASE define by the BL31_START symbol which is a relocatable and we
need to enable it by setting ENABLE_PIE := 1.

Signed-off-by: Marco Felsch <m.felsch@pengutronix.de>
Change-Id: I33c8e35c35112d70d2725eebe484a853a8aad9e0