RISC-V: defconfigs: Set CONFIG_FB=y, for FB console

We have CONFIG_FRAMEBUFFER_CONSOLE=y in the defconfigs, but that depends
on CONFIG_FB so it's not actually getting set.  I'm assuming most users
on real systems want a framebuffer console, so this enables CONFIG_FB to
allow that to take effect.

Fixes: 33c57c0d ("RISC-V: Add a basic defconfig")
Reviewed-by: Anup Patel <anup@brainfault.org>
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>