ARM i.MX27: Fix low reference clock path

The i.MX27 clock tree can either be driven from a 26MHz oscillator
or from a 32768Hz oscillator. The latter was not properly implemented,
the mux between these two pathes was missing. Add this mux and while
at it rename the 'prem' (premultiplier) clk to 'fpm' (Frequency
Pre-Multiplier) to better match the datasheet.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>