rtc: spear: initialize IRQ state before requesting alarm IRQ

devm_request_irq() enables the interrupt before it returns, so the handler
may run while probe is still initializing the device. Initialize the MMIO
address and spinlock before requesting the alarm IRQ.

Fixes: 0942a71e435f ("rtc: add support for spear rtc")
Cc: stable@vger.kernel.org
Assisted-by: Codex:GPT-5
Signed-off-by: Runyu Xiao <runyu.xiao@seu.edu.cn>
Link: https://patch.msgid.link/20260902080318.3498434-1-runyu.xiao@seu.edu.cn
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>