mtd: nand: gpmi: add gpmi_copy_bits function

Add a new function to copy bits (not bytes) from a memory region to
another one.
This function is similar to memcpy except it acts at bit level.
It is needed to implement GPMI raw access functions and adapt to the
hardware ECC engine which does not pad ECC bits to the next byte boundary.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Tested-by: Huang Shijie <shijie8@gmail.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>