drivers: rtc: sam: update for supporting RTC of sama7g5 devices

The basic characteristics of "atmel,sam-rtc" is the same as the ones
of sama7g5.

Differences between the RTCs listed bellow:
                    | atmel,sam-rtc | rtc in sama7g5
--------------------|---------------|----------------------
Persian calendar    |      Y        |         N
UTC Mode support    |      N        |         Y
Tamper Timestamping |      N        |         Y

Signed-off-by: Tony Han <tony.han@microchip.com>