refactor(plat/st): remove BL2 image loading

STM32MP1 does not use BL1, the loading of BL2 is done by ROM code. It is
then useless to have an entry BL2_IMAGE_ID in the policies.

Signed-off-by: Yann Gautier <yann.gautier@foss.st.com>
Change-Id: I464cedf588114d60522433123f8dbef32ae36818
1 file changed