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>