juno: Fix bug in plat_get_my_entrypoint

juno: Fix bug in plat_get_my_entrypoint

plat_get_my_entrypoint was branching to juno_do_reset_to_aarch_32_state,
which is not supposed to return, and in case of returning it implemented
an infinite loop. The problem was that plat_get_my_entrypoint was using
"b" instead of "bl", so juno_do_reset_to_aarch_32_state was returning to
the caller of plat_get_my_entrypoint instead of stop the system with a
panic.

To avoid this problem juno_do_reset_to_aarch_32_state was modified to
call directly to plat_panic_handler if it tries to return.

Change-Id: I591cf2dd78d27d8568fb15b91366e4b3dce027b5
Signed-off-by: Roberto Vargas <roberto.vargas@arm.com>