imx/clock-imx21: use a macro to define registers

This way the base address isn't hard coded in each register definition

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Cc: Sascha Hauer <kernel@pengutronix.de>
Cc: Russell King <linux@arm.linux.org.uk>
Cc: Holger Schurig <hs4233@mail.mn-solutions.de>
Cc: Rabin Vincent <rabin@rab.in>