[ARM] arm64: update arch_delay_loop_cycles()

The correct way for estimating machine cycle count from delay
loop count on Foundation v8 Model and Real HW seems to be:
Number of machine cycles = Loop count  * 2

This patch update arch_delay_loop_cycles() as per above.

In future, we will have CPU vendor specific ways of computing
machine cycles from delay loop count.

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