Utilize platform_get_irq_optional() to silence these messages:

brcmstb_thermal a581500.thermal: IRQ index 0 not found
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Link: https://lore.kernel.org/r/20220301181412.2008044-1-f.fainelli@gmail.comSigned-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>