ARM: clk: imx31: properly init clocks for machines with DT

Clock initialization for i.MX31 powered machines with DT support
should be done by a call of an init function registered with
CLK_OF_DECLARE() in common clock framework.

The change converts exported mx31_clocks_init_dt() into a static
initialization function registered by CLK_OF_DECLARE().

Signed-off-by: Vladimir Zapolskiy <vz@mleia.com>
Acked-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>