irq/qcom-mpm: Fix build error without MAILBOX

If MAILBOX is n, building fails:

drivers/irqchip/irq-qcom-mpm.o: In function `mpm_pd_power_off':
irq-qcom-mpm.c:(.text+0x174): undefined reference to `mbox_send_message'
irq-qcom-mpm.c:(.text+0x174): relocation truncated to fit: R_AARCH64_CALL26 against undefined symbol `mbox_send_message'

Make QCOM_MPM depends on MAILBOX to fix this.

Fixes: a6199bb5

 ("irqchip: Add Qualcomm MPM controller driver")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Acked-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20220317131956.30004-1-yuehaibing@huawei.com