rtc/spear: add Device Tree probing capability

rtc/spear: add Device Tree probing capability

SPEAr platforms now support DT and so must convert all drivers support DT.
This patch adds DT probing support for rtc and updates its documentation
too.

Signed-off-by: Viresh Kumar <[email protected]>
Cc: Stefan Roese <[email protected]>
Cc: Arnd Bergmann <[email protected]>
Cc: Alessandro Zummo <[email protected]>
Cc: Rajeev Kumar <[email protected]>
Cc: Rob Herring <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>