hw/mips: Add a bootloader helper

Add a bootloader helper to generate simple bootloaders for kernel.
It can help us reduce inline hex hack and also keep MIPS release 6
compatibility easier.

Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20210127065424.114125-2-jiaxun.yang@flygoat.com>
[PMD: Restricted bl_reg enum to C source,
      inverted bl_gen_write() args,
      added license in hw/mips/bootloader.h]
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>