iommu/amd: Add missing function prototypes to fix -Wmissing-prototypes

Few exported functions from AMD IOMMU driver are missing prototypes.
They have declaration in arch/x86/events/amd/iommu.h but this file
cannot be included in the driver.  Add prototypes to fix W=1 warnings
like:

    drivers/iommu/amd/init.c:3066:19: warning:
        no previous prototype for 'get_amd_iommu' [-Wmissing-prototypes]
     3066 | struct amd_iommu *get_amd_iommu(unsigned int idx)
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Link: https://lore.kernel.org/r/20200727183631.16744-1-krzk@kernel.orgSigned-off-by: Joerg Roedel <jroedel@suse.de>

drivers/iommu/amd/amd_iommu.h

@@ -41,6 +41,15 @@ extern int amd_iommu_guest_ir;
 struct iommu_domain;
 
 extern bool amd_iommu_v2_supported(void);
+extern struct amd_iommu *get_amd_iommu(unsigned int idx);
+extern u8 amd_iommu_pc_get_max_banks(unsigned int idx);
+extern bool amd_iommu_pc_supported(void);
+extern u8 amd_iommu_pc_get_max_counters(unsigned int idx);
+extern int amd_iommu_pc_get_reg(struct amd_iommu *iommu, u8 bank, u8 cntr,
+				u8 fxn, u64 *value);
+extern int amd_iommu_pc_set_reg(struct amd_iommu *iommu, u8 bank, u8 cntr,
+				u8 fxn, u64 *value);
+
 extern int amd_iommu_register_ppr_notifier(struct notifier_block *nb);
 extern int amd_iommu_unregister_ppr_notifier(struct notifier_block *nb);
 extern void amd_iommu_domain_direct_map(struct iommu_domain *dom);