xtensa: Provide dummy dma_alloc_attrs() and dma_free_attrs()

xtensa:allmodconfig fails to build with the following errors.

drivers/gpu/drm/nouveau/nvkm/subdev/instmem/gk20a.c:
	In function ‘gk20a_instobj_dtor_dma’:
drivers/gpu/drm/nouveau/nvkm/subdev/instmem/gk20a.c:154:2: error:
	implicit declaration of function ‘dma_free_attrs’
drivers/gpu/drm/nouveau/nvkm/subdev/instmem/gk20a.c:
	In function ‘gk20a_instobj_ctor_dma’:
drivers/gpu/drm/nouveau/nvkm/subdev/instmem/gk20a.c:218:2: error:
	implicit declaration of function ‘dma_alloc_attrs’

Xtensa does not provide those functions at this time.
Provide dummy implementations to avoid build errors.

Acked-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Chris Zankel <chris@zankel.net>