dm: wdt: Move s5p watchdog timer to drivers/watchdog/

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: David Müller <d.mueller@elsoft.ch>
Cc: Wolfgang Denk <wd@denx.de>
Cc: Albert Aribaud <albert.u.boot@aribaud.net>
Cc: U-Boot DM <u-boot-dm@lists.denx.de>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>