rtc: jz4740: Add support for JZ4760 SoC

rtc: jz4740: Add support for JZ4760 SoC

The WENR feature (set a magic value to enable RTC registers read-write)
first appeared on the JZ4760; the JZ4780 came much later.

Since it would be dangerous to specify a newer SoC's compatible string as
the fallback of an older SoC's compatible string, we add support for the
"ingenic,jz4760-rtc" compatible string in the driver.

This will permit to support the JZ4770 by having:
compatible = "ingenic,jz4770-rtc", "ingenic,jz4760-rtc";

Instead of doing:
compatible = "ingenic,jz4770-rtc", "ingenic,jz4780-rtc";

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Link: https://lore.kernel.org/r/20200311182318.22154-1-paul@crapouillou.net
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>