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/sama5d4ek/sama5d4ek.c b/board/atmel/sama5d4ek/sama5d4ek.c
index de4291f..357b223 100644
--- a/board/atmel/sama5d4ek/sama5d4ek.c
+++ b/board/atmel/sama5d4ek/sama5d4ek.c
@@ -386,7 +386,7 @@
 
 	/* enable MPDDR clock */
 	at91_periph_clk_enable(ATMEL_ID_MPDDRC);
-	writel(0x4, &pmc->scer);
+	writel(AT91_PMC_DDR, &pmc->scer);
 
 	/* DDRAM2 Controller initialize */
 	ddr2_init(ATMEL_BASE_MPDDRC, ATMEL_BASE_DDRCS, &ddr2);