Fix install of a missed header from debug_rom.

Fix install of a missed header from debug_rom.

The installed header files from the riscv subproject were incomplete, since
processor.h includes debug_rom_defines.h, and the latter was not installed.
Fix by moving it into riscv/, add it to the riscv subproject header list, which
ensures it will get installed.  While here, also add a missed dependency of debug_rom
on riscv/encoding.h to debug_rom/Makefile.