regulator: debugfs: Adding debugfs functions into regulator framework

Move regulator debugfs functions into core.c. This change allows the user
to read and edit regulator information in user space through the debugfs
file system.

Change-Id: I21fa2c1d5d4ab27871ebb543134c2cee5ad70ded
Signed-off-by: Brandon Leong <bleong@codeaurora.org>