ARM: vf610: ddrmc: do not write CR79 by default

The current value CTLUPD_AREF(0) is the reset value of the register,
so there is no need to write a value. If needed, the register can be
written using board specific CR settings.

Signed-off-by: Stefan Agner <stefan.agner@toradex.com>
Acked-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Reviewed-by: Lukasz Majewski <lukma@denx.de>