nvmem: core: Export nvmem cell info to userspace

Existing nvmem framework export full register space
as nvmem binary, but not exporting child node of nvmem
which is nvmem cell. Kernel can read the specific cell
by using nvmem_cell_read but userspace don't have such
provision.

Add framework to export nvmem cell as well, So
userspace can use it directly.

Change-Id: I3b41a91820f6d2a4536933e7bab4ac1b94f1541a
Signed-off-by: Shiraz Hashim <shashim@codeaurora.org>
Signed-off-by: Gaurav Kohli <gkohli@codeaurora.org>