Build error in UART driver for slstk3400a with device power management

[asmellby]

Describe the bug
Build error observed in weekly CI on Silabs Series 0 devices (slstk3400a).

To Reproduce
Build any application that uses serial/console with CONFIG_PM_DEVICE=y

Expected behavior
Build succeeds

Logs and console output

INFO    - 2) tests/kernel/device/kernel.device on slstk3400a/efm32hg322f64 error (Build failure)
INFO    - 3) tests/kernel/device/kernel.device.minimallibc on slstk3400a/efm32hg322f64 error (Build failure)
INFO    - 4) tests/kernel/device/kernel.device.pm on slstk3400a/efm32hg322f64 error (Build failure)
INFO    - 5) tests/lib/cpp/cxx/cpp.main.cpp14 on slstk3400a/efm32hg322f64 error (Build failure)
INFO    - 6) tests/lib/cpp/cxx/cpp.main.cpp17 on slstk3400a/efm32hg322f64 error (Build failure)
INFO    - 7) tests/lib/cpp/cxx/cpp.main.cpp20 on slstk3400a/efm32hg322f64 error (Build failure)
INFO    - 8) tests/lib/cpp/cxx/cpp.main.cpp2A on slstk3400a/efm32hg322f64 error (Build failure)
INFO    - 9) tests/lib/cpp/cxx/cpp.main.cpp2B on slstk3400a/efm32hg322f64 error (Build failure)
INFO    - 10) tests/lib/cpp/cxx/cpp.main.cpp98 on slstk3400a/efm32hg322f64 error (Build failure)

zephyr/drivers/serial/uart_gecko.c:500:58: error: 'USART_STATUS_TXIDLE' undeclared (first use in this function); did you mean 'USART_STATUS_TXBL'?
  500 |                 while (!(USART_StatusGet(config->base) & USART_STATUS_TXIDLE)) {
      |                                                          ^~~~~~~
      |                                                          USART_STATUS_TXBL
/home/mmahadevan/zephyrproject/zephyr/drivers/serial/uart_gecko.c:500:58: note: each undeclared identifier is reported only once for each function it appears in