iommu/riscv: Add shutdown function for iommu driver

This commit supplies shutdown callback for iommu driver. The shutdown
callback resets necessary registers so that newly booted kernel can pass
riscv_iommu_init_check() after kexec. Also, the shutdown callback resets
iommu mode to bare instead of off so that new kernel can still use PCIE
devices even when CONFIG_RISCV_IOMMU is not enabled.

Signed-off-by: Xu Lu <luxu.kernel@bytedance.com>
Link: https://lore.kernel.org/r/20250103093220.38106-3-luxu.kernel@bytedance.com


Signed-off-by: Joerg Roedel <jroedel@suse.de>