ARM: dts: pxa*: Fix up encoding of the /rtc interrupts property

This way the device tree validator learns that each cell of the property
constitutes a separate item. Otherwise it gets unnecessairly upset:

  pxa168-aspenite.dt.yaml: rtc@d4010000: interrupts: [[5, 6]] is too short
  pxa910-dkb.dt.yaml: rtc@d4010000: interrupts: [[5, 6]] is too short

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>