drivers: serial: silabs: Fix build for efm32wg_stk3800/efm32wg990f256

The gecko_uart.c driver supports both `silabs,gecko_uart` and
`silabs,gecko_usart` compat strings, however `PM_DEVICE_DT_INST_DEFINE()`
was missing when defining `uart` type instances. The
efm32wg_stk3800/efm32wg990f256 platform enables by default one
`gecko_uart` DT node and no `gecko_usart` nodes. This results in the
following build warning/error:

uart_gecko.c:673:12: warning: 'uart_gecko_pm_action' defined but not used

Add the missing call to `PM_DEVICE_DT_INST_DEFINE()`.

Signed-off-by: Johan Hedberg <johan.hedberg@silabs.com>