Allow to set not only the fractional divider, but also the prescaler for
the EMI clock in mxs_mem_init_clock(), and rename the parameters
accordingly to reflect the change. Port the existing board code to set
up the EMI clock explicitely with the old values. Also fix the
off-by-a-half error in the comments, which did not take the prescaler of
2 into account, on which the fractional divider is applied (according to
the i.MX23/i.MX28 Reference Manuals)

Signed-off-by: Roland Hieber <r.hieber@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>