clocksource: dw_apb_timer: Add common DTS glue for dw_apb_timer

Make a common device tree glue for clocksource/dw_apb_timer.
Move mach-picoxcell/time.c to be a generic device tree application
of the dw_apb_timer.

Configure mach-picoxcell to use the dw_apb_timer_of device tree
implementation in drivers/clocksource.

Signed-off-by: Pavel Machek <pavel@denx.de>
Signed-off-by: Dinh Nguyen <dinguyen@altera.com>
Acked-by: Jamie Iles <jamie@jamieiles.com>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>