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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>