mx5 clocks: Simplify imx_get_cspiclk()

The code handling the dividers was duplicated for each possible input clock, and
this function can benefit from the newly introduced get_standard_pll_sel_clk()
function instead of duplicating this mux handling code.

Signed-off-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>
Cc: Stefano Babic <sbabic@denx.de>