powerpc/52xx: Re-enable bestcomm driver in defconfigs

The bestcomm driver has been moved to drivers/dma, so to select
this driver by default additionally CONFIG_DMADEVICES has to be
enabled. Currently it is not enabled in the config despite existing
CONFIG_PPC_BESTCOMM=y in the config files. Fix it.

Signed-off-by: Anatolij Gustschin <agust@denx.de>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>