dmaengine: Remove dependency on MACH_BCM2708

Commit 96286b57

 ("dmaengine: Add support for BCM2835") added an
optional dependency on MACH_BCM2708. But there's no Kconfig symbol
MACH_BCM2708.

(There was an entry for MACH_BCM2708 in arch/arm/tools/mach-types from
v2.6.37 until v3.2. But it seems that entry was never used in the tree.)

This optional dependency can safely be removed.

Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>