mtd: onenand: Use a better name for samsung driver

Commit 55ed51ff ("{tty: serial, nand: onenand}: samsung: rename to
fix build warning") has changed the samsung.c driver to be
samsung_mtd.c in order to avoid a conflict in module names with the
tty driver.

Since the *_mtd suffix is very undescriptive, rename it to
onenand_samsung.c, following the folder's convention. Same will be
applied to the omap2 onenand driver.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>