linux-rpi: add common RPI RTC driver to kernel

@ncopa

Add driver for DS1307 and compatible RTCs (including DS3231) to kernel. This driver supports the majority of 3rd party RTCs addons available for Raspberry Pis.

Having the driver built into the kernel (rather than loaded by initramfs) ensures that the kernel will set the system clock from the RTC very early during boot, so preventing OpenRC "clock skew" messages from appearing.

Resolves #11889 (closed).