[ARM][gas] fix build breakage with gcc-10 by using correct enum type

Fixes

../../gas/config/tc-arm.c: In function 'parse_reg_list':
../../gas/config/tc-arm.c:1946:35: error: implicit conversion from 'enum reg_list_els' to 'enum arm_reg_type' [-Werror=enum-conversion]
 1946 |        reg = arm_reg_parse (&str, REGLIST_RN);
      |                                   ^~~~~~~~~~

gas/ChangeLog:

	* config/tc-arm.c (parse_reg_list): Use REG_TYPE_RN instead of
	REGLIST_RN.