mx5 clocks: Fix get_ipg_per_clk()

This fixes the "IPG PERCLK" frequency printed by the clocks command. The issue
was that get_ipg_per_clk() used periph_clk instead of lp_apm in the case
CCM.CBCMR.perclk_lp_apm_sel is set.

It also fixes I²C support.

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