Add support for custom reserved memory drivers. Call their init() function
for each reserved region and prepare for using operations provided by them
with by the reserved_mem->ops array.

Based on previous code provided by Josh Cartwright <joshc@codeaurora.org>
Signed-off-by: NMarek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: NGrant Likely <grant.likely@linaro.org>