drivers: serial: nrfx_uarte: Deprecate CONFIG_UART_x_NRF_HW_ASYNC

Deprecate mode which is using TIMER+(D)PPI for reliable byte counting.
Recently a new approach is added (CONFIG_UART_NRFX_UARTE_ENHANCED_RX)
which supports reliable byte counting without additional HW resource.
This mode is planned to be the only supported RX path mode.

Enhanced RX has slightly different behavior. There are no partial RX
packets (events with non-zero offset). There is UART_RX_BUF_RELEASED
after each UART_RX_RDY event.

Signed-off-by: Krzysztof Chruściński <krzysztof.chruscinski@nordicsemi.no>