issues with nrfx uarte hw async configuration validation

[pabigot]

When using nordic,nrf-uarte the following configuration:

CONFIG_UART_ASYNC_API=y
CONFIG_UART_0_NRF_HW_ASYNC=y

ends up referencing CONFIG_UART_0_NRF_HW_ASYNC_TIMER which by default is empty, so

zephyr/drivers/serial/uart_nrfx_uarte.c

Line 1394 in 8b6930e

CONFIG_UART_##idx##_NRF_HW_ASYNC_TIMER),)) \

produces a cascading build error starting with:

/mnt/nordic/zp/modules/hal/nordic/nrfx/drivers/include/nrfx_timer.h:62:39:
  error: 'NRF_TIMER' undeclared here (not in a function); did you mean 'NRF_TIMER2'?
   62 |     .p_reg            = NRFX_CONCAT_2(NRF_TIMER, id),             \
      |                                       ^~~~~~~~~

It would also be nice if the documentation made it more clear that (a) UARTE can hold received data for an unbounded time without signalling RX_RDY if NRF_HW_ASYNC isn't enabled, and (b) (resolved by #24209) enabling NRF_HW_ASYNC requires that an associated timer be identified.

Tooling to allow the user to identify a candidate timer without having to guess which ones are in use would also be welcome.

[pabigot]

This behavior occurs only when there is no hardware flow control. nope, not true. Occurs both with and without RTS/CTS.

[pabigot]

The bug part of this appears to be resolved by #24209 so I'm dropping that, leaving this as an enhancement to provide better diagnostics when CONFIG_UART_0_NRF_HW_ASYNC=y is provided without a companion RTC assignment like CONFIG_UART_0_NRF_HW_ASYNC_TIMER=4.