x86, io_apic: Introduce x86_io_apic_ops.disable()

This function pointer is used to call a system-specific
function for disabling the IO-APIC. Currently this is used
for IRQ remapping which has its own disable routine.

Also introduce the necessary infrastructure in the interrupt
remapping code to overwrite this and other function pointers
as necessary by interrupt remapping.

Signed-off-by: Joerg Roedel <joro@8bytes.org>
Acked-by: Sebastian Andrzej Siewior <sebastian@breakpoint.cc>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
diff --git a/arch/x86/include/asm/x86_init.h b/arch/x86/include/asm/x86_init.h
index 5769349..b1d2d6a 100644
--- a/arch/x86/include/asm/x86_init.h
+++ b/arch/x86/include/asm/x86_init.h
@@ -190,10 +190,11 @@
 };
 
 struct x86_io_apic_ops {
-	void		(*init)  (void);
-	unsigned int	(*read)  (unsigned int apic, unsigned int reg);
-	void		(*write) (unsigned int apic, unsigned int reg, unsigned int value);
-	void		(*modify)(unsigned int apic, unsigned int reg, unsigned int value);
+	void		(*init)   (void);
+	unsigned int	(*read)   (unsigned int apic, unsigned int reg);
+	void		(*write)  (unsigned int apic, unsigned int reg, unsigned int value);
+	void		(*modify) (unsigned int apic, unsigned int reg, unsigned int value);
+	void		(*disable)(void);
 };
 
 extern struct x86_init_ops x86_init;