[ARM] Rename generic-v8 board to generic board

There is nothing ARMv8 specific in generic-v8 board.

Also, generic-v8 board being fully device tree based hence
can be used for ARMv5, ARMv6, ARMv7, and ARMv7ve processors too.

This patch renames generic-v8 board to generic board so that
in future it can be used to 32bit ARM processors too.

Signed-off-by: Anup Patel <anup@brainfault.org>