watchdog: s3c2410_wdt: Use local register definitions

This patch moves register and bitfield definitions to the driver itself,
removing remaining dependencies on plat/ and mach/ headers.

Signed-off-by: Tomasz Figa <tomasz.figa@gmail.com>
Tested-by: Sylwester Nawrocki <sylvester.nawrocki@gmail.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>