board_f: Factor out bdinfo bi_mem{start, size} to setup_bdinfo
Move all assignments to gd->bd->bi_mem{start,size} to generic code in
setup_bdinfo.
Xtensa architecture is special in this regard as it defines its own
handling of gd->bd->bi_mem{start,size} fields. In order to avoid defining
a weak SDRAM function, let arch_setup_bdinfo overwrite the generic flags.
For ARC architecture, remove ARCH_EARLY_INIT_R from Kconfig since it is
not needed anymore.
Also, use gd->ram_base to populate bi_memstart to avoid an ifdef.