drivers: clock_control: gd32: initial support

This patch adds a clock control driver for GD32 platforms. It is
important to note that the driver is only able to handle peripheral
clocks, but not "system clocks" (e.g. PLL settings, SYS_CK, etc.).  On
some similar platforms (STM32) this task is embedded in the same clock
driver, performed at init time but with no options to do any
manipulation at runtime via the API calls. The clock control API as-is
is really orthogonal to "system clocks", and it is arguably a bad idea
to embed system clock init code in a clock control driver. It can be
done at SoC level still using Devicetree as a source of hardware
description/initial configuration.

Signed-off-by: Gerard Marull-Paretas <gerard@teslabs.com>