ACPI: RIMT: Fix unused function warnings when CONFIG_IOMMU_API is disabled

When CONFIG_IOMMU_API is disabled, some functions defined outside its
conditional scope become unused, triggering compiler warnings reported
by the kernel test robot.

Move these function definitions inside the #ifdef CONFIG_IOMMU_API block
to prevent unused function warnings when the configuration is disabled.

Fixes: 8f772955 ("ACPI: RISC-V: Add support for RIMT")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202509280031.8Sjkr4bh-lkp@intel.com/


Signed-off-by: Sunil V L <sunilvl@ventanamicro.com>
Link: https://lore.kernel.org/r/20251013181947.261759-1-sunilvl@ventanamicro.com