sun4i: Rename dram_clk_cfg to dram_clk_gate

The data sheet just calls it DRAM_CLK_REG, and on sun6i we've both a
dram_clk_cfg and dram_clk_gate, and the sun4i reg matches dram_clk_gate on
sun6i, so name it the same on sun4i.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Ian Campbell <ijc@hellion.org.uk>