Fix:OceanBase can not use 'PS' as table alias #888

[watchpoints]

What changes were proposed in this pull request?

https://github.com/oceanbase/oceanbase/issues/888

• sql_parser_mysql_mode.y
• add ps as non_reserved_keyword

Why are the changes needed?

MySQL [(none)]> SELECT * FROM oceanbase.gv$partition ps limit 1; ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your OceanBase version for the right syntax to use near 'ps limit 1' at line 1

Will break the compatibility? How if so?

NO

Does this PR introduce any user-facing change?

How was this patch tested?

Checklist

• [ ] I've run the tests to see all new and existing tests pass.

• [ ] obclient select * from user as ps limit 4; is ok