The warm boot mailbox code is compiled if PROGRAMMABLE_RESET_ADDRESS
is disabled.

The warm boot mailbox is useless for UniPhier SoC family because BL1
is not the first image.  The UniPhier platform implements non-TF ROM,
then BL1 works as a pseudo ROM, so it is never executed in the warm
boot.

The reset vector address is not actually programmable for UniPhier
platform, but it should not hurt to enable PROGRAMMABLE_RESET_ADDRESS
to disable the mailbox and remove pointless plat_get_my_entrypoint.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>