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/at91sam9x5ek/at91sam9x5ek.c b/board/atmel/at91sam9x5ek/at91sam9x5ek.c
index 0455e2c..1738a2b 100644
--- a/board/atmel/at91sam9x5ek/at91sam9x5ek.c
+++ b/board/atmel/at91sam9x5ek/at91sam9x5ek.c
@@ -353,7 +353,7 @@
 	ddr2_conf(&ddr2);
 
 	/* enable DDR2 clock */
-	writel(0x4, &pmc->scer);
+	writel(AT91_PMC_DDR, &pmc->scer);
 
 	/* Chip select 1 is for DDR2/SDRAM */
 	csa = readl(&matrix->ebicsa);