mtd: rawnand: gpmi: move all driver code into single file

This moves the whole driver into a single C file. The filename gpmi-lib
implies that it implements library functions, but in fact there are
several cases where functions in gpmi-lib.c call back into functions in
gpmi-nand.c. With this one has to constantly jump between those two
files, so moving it into a single file improves readability, even when
the file gets quite large.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Reviewed-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>