arm: socfpga: gen5: don't zero bss in board_init_f()

The socfpga gen5 SPL manually zeroed bss in board_init_f(). Now that the
DDR driver does not use bss any more, bss is not used before board_init_r()
and we can remove this hack.

bss is normally zeroed by crt0.S, but after board_init_f(), before
board_init_r(). socfpga just had this double-zeroing because it invalidly
used bss in board_init_f() already (during DDR initialization).

Signed-off-by: Simon Goldschmidt <simon.k.r.goldschmidt@gmail.com>