MIPS, clk: move jz4740 clock suspend, resume functions to jz4740-cgu
The jz4740-cgu driver already has access to the CGU, so it makes sense
to move the few remaining accesses to the CGU from arch/mips/jz4740
there too. Move the jz4740_clock_{suspend,resume} functions there for
such consistency. The arch/mips/jz4740/clock.c file now contains nothing
more of use & so is removed.