target-arm: Pull "add one cpreg to hashtable" into its own function

define_one_arm_cp_reg_with_opaque() has a set of nested loops which
insert a cpreg entry into the hashtable for each of the possible
opc/crn/crm values allowed by wildcard specifications. We're about
to add an extra loop to this nesting, so pull the core of the loop
(which adds a single entry to the hashtable) out into its own
function for clarity.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <rth@twiddle.net>
