[ARM] Add support for ARMv5 boards in generic board support

The patch completes the generic board support by allowing ARMv5 boards
using generic board support.

As part of this patch, we also rename arch/arm/cpu/arm32/cpu_proc_arm926.S
to arch/arm/cpu/arm32/cpu_proc_v5.S so that it can be used for both ARM926
and Generic-v5 CPUs.

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