mx5 clocks: Fix get_periph_clk()

In the case periph_clk comes from periph_apm_clk, the latter is selected by the
CCM.CBCMR.periph_apm_sel mux, which can source the lp_apm clock from its
input ♯2. get_periph_clk() returned 0 instead of the lp_apm clock frequency in
this case.

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