irqchip/irq-bcm7120-l2: Switch to IRQCHIP_PLATFORM_DRIVER

Allow the user selection and building of this interrupt controller
driver as a module since it is used on ARM/ARM64 based systems as a
second level interrupt controller hanging off the ARM GIC and is
therefore loadable during boot.

To avoid using of_irq_count() which is not exported towards module,
switch the driver to use the platform_device provided by the irqchip
platform driver code and resolve the number of interrupts using
platform_irq_count().

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20211020184859.2705451-11-f.fainelli@gmail.com