riscv: align bootm implementation with that of other architectures

The bootm implementation of RISC-V diverges from that of other
architectures. Update it to match the implementation of other
architectures. The ARM implementation is used as a reference.

This adds the following features and changes to RISC-V.
* Add support for the BOOTM_STATE_OS_FAKE_GO command
* Call the remove function on devices with the removal flag set before
booting Linux
* Force disconnect USB devices from the host before booting Linux
* Print and add bootstage information to the device tree before booting
Linux

Signed-off-by: Lukas Auer <lukas.auer@aisec.fraunhofer.de>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>