clk: imx: rename clk-imx51-imx53.c to clk-imx5.c

As the driver is handling all i.MX5 series SoCs inlcuding i.MX50, rather
than just i.MX51 and i.MX53, let's rename it to clk-imx5.c.

Signed-off-by: Shawn Guo <shawnguo@kernel.org>
Reviewed-by: Stephen Boyd <sboyd@kernel.org>