docs: re-parent BL2 platform hooks for measured boot

bl2_plat_mboot_init/finish() functions documentation was incorrectly
hooked up to BL2U-specific section.

Change-Id: I758cb8142e992b0c85ee36d5671fc9ecd5bde29b
Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>
1 file changed