feat(stm32mp2): boot BL33 at EL2 by default

Set BL33 to boot at EL2 by default. The STM32MP_BL33_EL1 must be
manually enabled to boot at EL1.
While at it, document the flag in STM32MP2 documentation.

Change-Id: I9c5a183553c5b2f54c55877ecb7b02e04cb333e6
Signed-off-by: Maxime Méré <maxime.mere@foss.st.com>