riscv: bootm: Add dm_remove_devices_flags() call to do_bootm_linux()

This adds a call to dm_remove_devices_flags() to do_bootm_linux()
so that drivers that have one of the removal flags set (e.g.
DM_FLAG_ACTIVE_DMA_REMOVE) in their driver struct, may do some
last-stage cleanup before the OS is started.

arm and x86 already did such, and we should do the same for riscv.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>