ARM: s3c64xx: switch to generic watchdog driver reset

Similarly to commit f6361c6b ("ARM: S3C24XX: remove separate restart
code"), the platform watchdog reset code can be removed in favor of
a generic watchdog driver which already handles reset.

This allows removal of a bunch of machine code and fixes also W=1
compile warnings:

    arch/arm/plat-samsung/watchdog-reset.c:29:6: warning: no previous prototype for 'samsung_wdt_reset' [-Wmissing-prototypes]
    arch/arm/plat-samsung/watchdog-reset.c:69:13: warning: no previous prototype for 'samsung_wdt_reset_of_init' [-Wmissing-prototypes]
    arch/arm/plat-samsung/watchdog-reset.c:89:13: warning: no previous prototype for 'samsung_wdt_reset_init' [-Wmissing-prototypes]

The generic watchdog-based system reset is not exactly the same as
before.  The previous method had a fallback to soft_restart() which now
is gone.

The commit also removes a FIXME note about calling s3c64xx_clk_init()
inside s3c64xx_init_irq().  No one fixed this since long time and the
note is not meaningful anymore because watchdog part is removed.

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Reviewed-by: Tomasz Figa <tomasz.figa@gmail.com>