docs/specs/riscv-iommu.rst: add HPM support info

Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20250224190826.1858473-12-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>

docs/specs/riscv-iommu.rst

@@ -82,6 +82,8 @@ Several options are available to control the capabilities of the device, namely:
 - "off" (Out-of-reset translation mode: 'on' for DMA disabled, 'off' for 'BARE' (passthrough))
 - "s-stage": enable s-stage support
 - "g-stage": enable g-stage support
+- "hpm-counters": number of hardware performance counters available. Maximum value is 31.
+  Default value is 31. Use 0 (zero) to disable HPM support
 
 riscv-iommu-sys device
 ----------------------