mx5 clocks: Fix get_lp_apm()

If CCM.CCSR.lp_apm is set, the lp_apm clock is not necessarily 32768 Hz x 1024.
In that case:
 - on i.MX51, this clock comes from the output of the FPM,
 - on i.MX53, this clock comes from the output of PLL4.

This patch fixes the code accordingly.

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