android: ion: add ion_iovmm_[un]map()

android: ion: add ion_iovmm_[un]map()

iovmm_map() provided by exynos-iovmm is time-consuming. If the same
buffer is iovm mapped to a device, iovmm_map() does the same thing
with much time. iovmm_map() is not able to avoid this repeat because
it has no information to identify a buffer. Thus, ION provides the
avoidance of repeating mapping the same buffer.

Change-Id: I4c62f615dca6dfb5f8d2fe366ccc0a89af1d485e
Signed-off-by: Cho KyongHo <pullip.cho@samsung.com>