rtc: mxc: use spin_lock_irqsave instead of spin_lock_irq in IRQ context

As spin_unlock_irq will enable interrupts.
mxc_rtc_irq_enable is called from interrupt handler mxc_rtc_interrupt.
Interrupts are enabled in interrupt handler.
Use spin_lock_irqsave/spin_unlock_irqrestore instead of spin_(un)lock_irq
in IRQ context to avoid this.

Signed-off-by: Fuqian Huang <huangfq.daxian@gmail.com>
Link: https://lore.kernel.org/r/20190807082310.10135-1-huangfq.daxian@gmail.com
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>