clk_k210.c: Clean up how we handle nop

Now that sandbox has <asm/barrier.h> and defines nop() there we should
include that in our driver for clarity and then remove our local nop()
from <k210/pll.h>.

Reviewed-by: Sean Anderson <seanga2@gmail.com>
Signed-off-by: Tom Rini <trini@konsulko.com>