mtd: onenand/samsung: make regs-onenand.h file local

Nothing uses the NAND register definitions other than the
actual driver, so we can move the header file into the
same local directory, which lets us build it in a multiplatform
configuration.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Kyungmin Park <kyungmin.park@samsung.com>
Cc: linux-mtd@lists.infradead.org
Cc: David Woodhouse <dwmw2@infradead.org>