arm: add missing include platform-data/atmel.h

Include corresponding headerfile <linux/platform-data/atmel.h> for
function at91_suspend_entering_slow_clock().

../arch/arm/mach-at91/pm.c:279:5: warning: no previous prototype for ‘at91_suspend_entering_slow_clock’ [-Wmissing-prototypes]
 int at91_suspend_entering_slow_clock(void)
     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Signed-off-by: Philippe Mazenauer <philippe.mazenauer@outlook.de>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

arch/arm/mach-at91/pm.c

@@ -19,6 +19,7 @@
 #include <linux/suspend.h>
 
 #include <linux/clk/at91_pmc.h>
+#include <linux/platform_data/atmel.h>
 
 #include <asm/cacheflush.h>
 #include <asm/fncpy.h>