ARM: mx5: clock-imx51: make *ipg clocks secondary clocks of their corresponding peripheral clocks

Currently the uarts and timer only work because they are
turned on by reset default. Make them secondary clocks
of their corresponding peripheral clocks to make sure they
are turned on when necessary. Also, register some clocks
to get rid of compiler warnings

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>