mx5 clocks: Fix get_uart_clk()

This function returned 66500000 instead of the correct lp_apm clock frequency if
the CCM.CSCMR1.uart_clk_sel mux is set to 3.

This patch fixes this issue by introducing the get_standard_pll_sel_clk()
function that will be used by future patches to handle identical muxes used by
many other clocks.

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