mx25: properly initialize clocks

This patch disables all unnecessary clock in mx25_clocks_init() to make a clean
start, the same as is being done for the rest of the i.MX chips.

This patch was tested on i.MX25 PDK.

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