fix(a3k): fix comment about BootROM address range

A53 AP BootROM is just 16 kB long and is mapped to address range
0xFFFF0000-0xFFFF4000. RVBAR_EL3 register has value 0xFFFF0000.
A53 AP BootROM itself is in the BootROM window which is 1 MB long and
mapped to address range 0xFFF00000-0xFFFFFFFF.

CM3 BootROM is not accessible from A53 core.

Signed-off-by: Pali Rohár <pali@kernel.org>
Change-Id: I5d4a4c7b1e7550c4738c67a872d341f945d48bbc