nios2: Fix unused variable warning

Fix the following compiler splat by adding __maybe_unused annotation to
the variable. Using this particular annotation has the least ugly impact
on the code compared to using ifdeffery.

arch/nios2/kernel/setup.c: In function 'nios2_boot_init':
arch/nios2/kernel/setup.c:107:7: warning: unused variable 'cmdline_passed' [-Wunused-variable]
  char cmdline_passed[COMMAND_LINE_SIZE] = { 0, };
       ^

Signed-off-by: Marek Vasut <marex@denx.de>
Acked-by: Ley Foon Tan <lftan@altera.com>