mx25: add a comment documenting undocumented IPG clocks

The information in the i.MX25 Reference Manual is lacking. Add information from
the Freescale BSP.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Acked-by: Wolfram Sang <w.sang@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>