mfd: syscon: Add atmel system timer registers definition

AT91RM920 has a memory range reserved for timer and watchdog configuration.
Expose those registers so that drivers can make use of the system timer syscon
declared in at91 DTs.

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Acked-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>