boards: silabs: efr32_radio: Add USART pinctrl configuration node

this commit introduces a usart pinctrl node to enable the spi functionality
in the efr32_radio board. the location* properties have been deprecated
so pinctrl support has been added in the device tree included file.

Before this commit (using deprecated location properties):
- usart@40010400 (DISABLED)

After this commit (using pinctrl properties):
- usart@40010400 (READY)

Signed-off-by: Arunmani Alagarsamy <arunmani.a@capgemini.com>