clk: sunxi: Add DRAM gates support for sun4i-a10

The A10/A20 share the same set of DRAM clock gates, which controls
direct memory access for some peripherals.

On the A10, bit 15 controls the system's DRAM clock output (possibly
to the DRAM chips), which we need to keep on.

On the A20 this has been moved to the DRAM controller, becoming a no-op.
However it is still listed in the user manual, so add it anyway.

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