boards: gd32e507z_eval: add initial support

Add initial support for the GD32E507Z-EVAL board. Only basic peripherals
are enabled for now:

- UART
- GPIO (LEDs, keys)
- PWM

Others, e.g. I2C, EEPROM, DAC, etc. require further testing and likely
some driver adjustments.

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