board: atmel: clean up peripheral clock code

Due to introducing the new peripheral clock handle functions,
use these functions to reduce duplicated code.

Signed-off-by: Wenyou Yang <wenyou.yang@atmel.com>
Tested-by: Heiko Schocher <hs@denx.de>
[Rebased on current master, fixup for at91rm9200ek]
Signed-off-by: Andreas Bießmann <andreas.devel@googlemail.com>
diff --git a/board/atmel/sama5d3xek/sama5d3xek.c b/board/atmel/sama5d3xek/sama5d3xek.c
index 0d824fc..8576560 100644
--- a/board/atmel/sama5d3xek/sama5d3xek.c
+++ b/board/atmel/sama5d3xek/sama5d3xek.c
@@ -443,14 +443,13 @@
 
 void mem_init(void)
 {
-	struct at91_pmc *pmc = (struct at91_pmc *)ATMEL_BASE_PMC;
 	struct atmel_mpddrc_config ddr2;
 
 	ddr2_conf(&ddr2);
 
-	/* enable MPDDR clock */
+	/* Enable MPDDR clock */
 	at91_periph_clk_enable(ATMEL_ID_MPDDRC);
-	writel(AT91_PMC_DDR, &pmc->scer);
+	at91_system_clk_enable(AT91_PMC_DDR);
 
 	/* DDRAM2 Controller initialize */
 	ddr2_init(ATMEL_BASE_MPDDRC, ATMEL_BASE_DDRCS, &ddr2);