doc: Fix RSE oriented boot flow diagram

Change diagram to show that RSE BL1_2 is executed from SRAM, because
the current diagram suggests that BL1_2 is executed from OTP memory,
which is not the case.
Additionally, change diagram to show that BL2 releases SCP RAMFW from
reset, because the diagram suggests that this is done by RSE Runtime.

Signed-off-by: Maximilian Berndt <maximilian.berndt@arm.com>