driver-core: platform: Add platform_irq_count()

A recent patch added calls to of_irq_count() in the qcom pinctrl
drivers and that caused module build failures because
of_irq_count() is not an exported symbol. We shouldn't export
of_irq_count() to modules because it's an internal OF API that
shouldn't be used by drivers. Platform drivers should use
platform device APIs instead. Therefore, add a platform_irq_count()
API that mirrors the of_irq_count() API so that platform drivers
can stay DT agnostic.

Cc: Andy Gross <andy.gross@linaro.org>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>