powerpc/mpc85xx: socrates: Re-enable building u-boot-socrates.bin

U-Boot build system builds final U-Boot binary for socrates board in custom
file u-boot-socrates.bin (instead of standard u-boot.bin). Output target
file u-boot-socrates.bin is generated by binman as defined in board binman
config file arch/powerpc/dts/socrates-u-boot.dtsi.

But binman was disabled in commit 5af42eaf ("Makefile: Reduce usage of
custom mpc85xx u-boot.bin target") for all mpc85xx boards which do not use
standard powerpc binman config file arch/powerpc/dts/u-boot.dtsi and boards
which do not require binman at all.

The only such mpc85xx board is socrates. So since that commit, U-Boot does
not final binary for socrates board anymore.

Fix this issue by re-enabling binman for socrates board. And build process
starts again producing u-boot-socrates.bin binary.

Note that build process for this socrates board always produce u-boot.bin
binary which is broken and not usable for socrates board. Long term
solution should be to disable building broken binary u-boot.bin and then
renaming u-boot-socrates.bin to u-boot.bin, or switching to use common
powerpc binman config file arch/powerpc/dts/socrates-u-boot.dtsi (if it is
possible).

Fixes: 5af42eaf ("Makefile: Reduce usage of custom mpc85xx u-boot.bin target")
Signed-off-by: Pali Rohár <pali@kernel.org>
Tested-by: Heiko Schocher <hs@denx.de>