clocksource/drivers/sh_mtu2: Do not loop using platform_get_irq_by_name()

As platform_get_irq_by_name() now prints an error when the interrupt
does not exist, looping over possibly non-existing interrupts causes the
printing of scary messages like:

    sh_mtu2 fcff0000.timer: IRQ tgi1a not found
    sh_mtu2 fcff0000.timer: IRQ tgi2a not found

Fix this by using the platform_irq_count() helper, to avoid touching
non-existent interrupts.  Limit the returned number of interrupts to the
maximum number of channels currently supported by the driver in a
future-proof way, i.e. using ARRAY_SIZE() instead of a hardcoded number.

Fixes: 7723f4c5 ("driver core: platform: Add an error message to platform_get_irq*()")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20191016143003.28561-1-geert+renesas@glider.be