serial: xilinx: uartlite: update irq_enable to use DT_INST_IRQN_BY_IDX

Update the irq_enable macro to use the DT_INST_IRQN_BY_IDX helper.

This ensures proper handling of IRQ numbers in systems with multi-level
interrupt configurations.

Signed-off-by: Ajay Neeli <ajay.neeli@amd.com>