irqchip: Place CONFIG_SIFIVE_PLIC into the menu

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

Fixes: 8237f8bc4f ("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

drivers/irqchip/Kconfig

@@ -487,8 +487,6 @@ config TI_SCI_INTA_IRQCHIP
 	  If you wish to use interrupt aggregator irq resources managed by the
 	  TI System Controller, say Y here. Otherwise, say N.
 
-endmenu
-
 config SIFIVE_PLIC
 	bool "SiFive Platform-Level Interrupt Controller"
 	depends on RISCV
@@ -500,3 +498,5 @@ config SIFIVE_PLIC
 	   interrupt sources are subordinate to the PLIC.
 
 	   If you don't know what to do here, say Y.
+
+endmenu