aarch64: Apply narrowing of allowed immediate values for SYSP

aarch64: Apply narrowing of allowed immediate values for SYSP

While CRn and CRm fields in the SYSP instruction are 4-bit wide and
are thus able to accommodate values in the range 0-15, the
specifications for the SYSP instructions limit their ranges to 8-9 for
CRm and 0-7 in the case of CRn.

This led to the need to signal in some way to the operand parser that
a given operand is under special restrictions regarding its use.  This
is done via the new `F_OPD_NARROW' flag, indicating a narrowing in the
range of operand values for fields in the instruction tagged with the
flag.

The flag is then used in `parse_operands' when the instruction is
assembled, but needs not be taken into consideration during
disassembly.