irqchip: Place CONFIG_SIFIVE_PLIC into the menu

Somehow CONFIG_SIFIVE_PLIC ended up outside of the "IRQ chip support"
menu.

Fixes: 8237f8bc ("irqchip: add a SiFive PLIC driver")
Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: Palmer Dabbelt <palmer@sifive.com>
Acked-by: Palmer Dabbelt <palmer@sifive.com>
Link: https://lore.kernel.org/r/20191002144452.10178-1-j.neuschaefer@gmx.net