watchdog: s5p_wdt: Include missing CPU header

s5p watchdog driver calls samsung_get_base_watchdog() function, but its
prototype is not included. That might lead to build warnings like this:

    drivers/watchdog/s5p_wdt.c: In function 'wdt_stop':
    drivers/watchdog/s5p_wdt.c:16:26:
        warning: implicit declaration of function
        'samsung_get_base_watchdog' [-Wimplicit-function-declaration]
             16 |   (struct s5p_watchdog *)samsung_get_base_watchdog();
                |                          ^~~~~~~~~~~~~~~~~~~~~~~~~

Include asm/arch/cpu.h to fix that issue.

Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>