clk: ast2600: Add RMII RCLK gates for all four MACs

RCLK is a fixed 50MHz clock derived from HPLL/HCLK that is described by a
single gate for each MAC.

Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Link: https://lkml.kernel.org/r/20191010020725.3990-3-andrew@aj.id.au


Reviewed-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>