nios2: clean up macros that do not need a value in board header

Clean up macros that do not need a value as suggested by
Marek.

Signed-off-by: Thomas Chou <thomas@wytron.com.tw>
Tested-by: Marek Vasut <marex@denx.de>
Acked-by: Marek Vasut <marex@denx.de>