irqchip/renesas-intc-irqpin: Merge irlm_bit and needs_irlm

Get rid of the separate flag to indicate if the IRLM bit is present in
the INTC/Interrupt Control Register 0, by considering -1 an invalid
irlm_bit value.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20201028153955.1736767-1-geert+renesas@glider.be