drivers: uart: uart_cmsdk_apb: fix interrupt handling

The CMSDK uart interrupts for TX and RX can either be treated as a
signel interrupt line or distinct interrupts for TX & RX.  In the case
that they were distinct we didn't get the ifdef correct based on DTS.

If we have 2 interrupts in DTS we assume they are for TX & RX and thus
build the interrupt support for distinct TX & RX ISRs.

Also, cleanup handling of UART_2..UART_4 to be similar to how
UART_0/UART_1 code is using DT_INST_IRQN(x).

Fixes #30770
Fixes #25601

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>