Correctly parse register values provided by the monitor.

Correctly parse register values provided by the monitor.
* rom68k-rom.c: #include "value.h".
(is_hex_digit, hex_digit_value, is_whitespace,
rom68k_supply_one_register): New static functions.
(rom68k_supply_register): Call rom68k_supply_one_register, instead
of monitor_supply_register; the latter was incorrectly parsing
the values.
* Makefile.in (rom68k-rom.o): Note that this now #includes value.h.