clk: sunxi-ng: mult: Support PLL lock detection

Some PLL clocks are N (multiplier) type clocks, or can be simplified
as such. An example of the former is the DDR1 PLL clock on the A33.
An example of the latter is the CPU PLL clock on the A80, in which
the P divider is only used for low frequencies that are of little
use. Both clocks support PLL lock detection.

The mult clock macro implies support for this, but that is not true.
The field is simply discarded. This patch adds proper support for it.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>