Previously, picosoc.v needed to be sourced before picorv32.v to
ensure that the PICORV32_REGS `define (used to select implementation
for the register file) was set to picosoc_regs
This allows for overriding PICORV32_REGS, e.g. by setting it
externally in the EDA tool invocation. In this case, the compile
order between picorv32.v and picosoc.v is not important.
Note: This change will break the safety check if PICORV32_REGS
is defined between sourcing picorv32.v and picosoc.v
Clifford Wolf
Olof Kindgren