ARM: enable ARM_HAS_SG_CHAIN for multiplatform

Enable ARM_HAS_SG_CHAIN for all multiplatform targets, it makes sense
to enable on all "modern" platforms; downsides are limited for platforms
that don't need it.

Requested-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Olof Johansson <olof@lixom.net>