nios2: add clear and set bits macros

These macros can be used to clear and set multiple bits
in a register using a single call.

Signed-off-by: Thomas Chou <thomas@wytron.com.tw>
Reviewed-by: Marek Vasut <marex@denx.de>
Acked-by: Chin Liang See <clsee@altera.com>