ARM: dts: at91: mpa1600: remove leftovers clock definition

The clocks group properties and the clock@0 node have been obsolete for two
years, remove them.

Cc: Joachim Eastwood <manabian@gmail.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>