arm: at91: pmc: replace the constant with a define in at91_pmc.h

To enable the clocks on the at91 boards a constant (0x4) is used.
This is replaced with a define in at91_pmc.h (1 <<  2).

Signed-off-by: Erik van Luijk <evanluijk@interact.nl>
Reviewed-by: Andreas Bießmann <andreas.devel@googlemail.com>
diff --git a/board/atmel/at91sam9m10g45ek/at91sam9m10g45ek.c b/board/atmel/at91sam9m10g45ek/at91sam9m10g45ek.c
index d2ade4d..2fea56f 100644
--- a/board/atmel/at91sam9m10g45ek/at91sam9m10g45ek.c
+++ b/board/atmel/at91sam9m10g45ek/at91sam9m10g45ek.c
@@ -136,7 +136,7 @@
 	ddr2_conf(&ddr2);
 
 	/* enable DDR2 clock */
-	writel(0x4, &pmc->scer);
+	writel(AT91_PMC_DDR, &pmc->scer);
 
 	/* DDRAM2 Controller initialize */
 	ddr2_init(ATMEL_BASE_DDRSDRC0, ATMEL_BASE_CS6, &ddr2);