ARM: samsung: make SAMSUNG_DMADEV optional

The only remaining driver using the samsung dmadev code is the broken
samsung-ac97 sound driver. However, as found by Russell's autobuilder,
the elaborate dependency chains around it cause problems with
circular dependencies.

This is an attempt to simplify those dependencies by making the
SAMSUNG_DMADEV option user-selectable. I also try to keep the
default settings for all related options unchanged, so we don't
introduce any regressions against earlier testing on linux-next.

In particular, all s3c64xx and s5p* platforms keep selecting the
pl330 and pl08x drivers they require, but the select statement
is now moved towards the main platform option, and it remains
optional by unselecting CONFIG_DMADEVICES.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Kukjin Kim <kgene.kim@samsung.com>