riscv: enable allwinner RTC

[mhorne]

Summary

For the Allwinner D1 (Nehza) SBC.

This RTC driver is also a clock provider, which registers two fixed
clocks. In all the devices we currently support, the names of the clocks
are present in the "clock-output-names" property of the device tree.
This is not the case for the D1 DTS, as this property does not appear in
upstream. Therefore the clock definitions are statically assigned a
name, which is overridden when specified.

[mhorne]

@manu This is the first of several changes enabling Allwinner drivers on the riscv platform (probably not all coming today). It suggests that we might move these sources to a more common place (out of arm subdir), but I do not really see any reason this must happen first.

[manu]

In D47514#1083739, @mhorne wrote:

@manu This is the first of several changes enabling Allwinner drivers on the riscv platform (probably not all coming today). It suggests that we might move these sources to a more common place (out of arm subdir), but I do not really see any reason this must happen first.

Sure, that's why I started moving a lot of other drivers too in the past.