ARM: SAMSUNG: Remove unused plat/regs-watchdog.h header

Since there are no remaining users of this header, it can be safely
dropped.

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>