riscv: bootm: Support to boot riscv-linux

riscv-linux should use BBL (Berkeley bootloader) for
loading the Linux kernel.

U-Boot can play as FSBL(first stage bootloader)
to boot BBL and riscv-linux.

In BBL's init_first_hart(), it will pass dtb with a1.
So implement bootm to pass arguments to BBL correctly.

Signed-off-by: Rick Chen <rick@andestech.com>
Signed-off-by: Rick Chen <rickchen36@gmail.com>