lib/genalloc: add a helper function for DMA buffer allocation

When using pool space for DMA buffer, there might be duplicated calling of
gen_pool_alloc() and gen_pool_virt_to_phys() in each implementation.

Thus it's better to add a simple helper function, a compatible one to the
common dma_alloc_coherent(), to save some code.

Signed-off-by: Nicolin Chen <b42378@freescale.com>
Cc: "Hans J. Koch" <hjk@hansjkoch.de>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Eric Miao <eric.y.miao@gmail.com>
Cc: Grant Likely <grant.likely@linaro.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Haojian Zhuang <haojian.zhuang@gmail.com>
Cc: Jaroslav Kysela <perex@perex.cz>
Cc: Kevin Hilman <khilman@deeprootsystems.com>
Cc: Liam Girdwood <lgirdwood@gmail.com>
Cc: Mark Brown <broonie@kernel.org>
Cc: Mauro Carvalho Chehab <m.chehab@samsung.com>
Cc: Rob Herring <rob.herring@calxeda.com>
Cc: Russell King <linux@arm.linux.org.uk>
Cc: Sekhar Nori <nsekhar@ti.com>
Cc: Takashi Iwai <tiwai@suse.de>
Cc: Vinod Koul <vinod.koul@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>