hw/riscv/riscv-iommu: add riscv_iommu_hpm_incr_ctr()

hw/riscv/riscv-iommu: add riscv_iommu_hpm_incr_ctr()

This function will increment a specific counter, generating an interrupt
when an overflow occurs.

Some extra changes in riscv-iommu.c  were required to add this new
helper in riscv-iommu-hpm.c:

- RISCVIOMMUContext was moved to riscv-iommu.h, making it visible in
  riscv-iommu-hpm.c;

- riscv_iommu_notify() is now public.

No behavior change is made since HPM support is not being advertised
yet.

Signed-off-by: Tomasz Jeznach <tjeznach@rivosinc.com>
Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Acked-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20250224190826.1858473-5-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>