uniphier: switch to BL2-AT-EL3 and remove BL1 support

UniPhier platform implements non-TF boot ROM.  Prior to the BL2-AT-EL3
support, BL1 (worked as a pseudo ROM) was needed just for ensuring BL2
is entered at EL1-S.  Now, this platform is able to avoid this waste.

Enable the BL2_AT_EL3 option, and remove BL1.

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