ARM/clocksource: use automatic DT probing for ux500 PRCMU

The ARM core kernel already calls clocksource_of_init() so why
go to all the trouble of locating and probing this node in the
machine. CLOCKSOURCE_OF_DECLARE() will take care of it in the
clocksource driver, and thus we can also get rid of the
dangling header file <linux/clksrc-dbx500-prcmu.h>

Suggested-by: Arnd Bergmann <arndb@linaro.org>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>