drivers: serial: pl011: prevent concurrent interrupt callback execution

Since the callback can be executed from either software or a hardware
interrupt, it's possible for multiple instances of the callback function
to run concurrently. That should not happen and can be particularly
problematic on SMP systems. This commit adds a spinlock to prevent
concurrent execution.

This addresses a Twister CI failure discovered in #85539.

Signed-off-by: Xudong Zheng <7pkvm5aw@slicealias.com>