dmaengine: dma-jz4780: Don't depend on MACH_JZ4780

If we make this driver depend on MACH_JZ4780, that means it can be
enabled only if we're building a kernel specially crafted for a
JZ4780-based board, while most GNU/Linux distributions will want one
generic MIPS kernel that works on multiple boards.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Signed-off-by: Vinod Koul <vkoul@kernel.org>