board: fsl: lx2160a: unused variable gic_lpi_base

If the board is configured without CONFIG_GIC_V3_ITS, an error occurs:

board/freescale/lx2160a/lx2160a.c: In function ‘ft_board_setup’:
board/freescale/lx2160a/lx2160a.c:673:6: error: unused variable
‘gic_lpi_base’ [-Werror=unused-variable]
  673 |  u64 gic_lpi_base;
      |      ^~~~~~~~~~~~

Let's define the variable as __maybe_unused.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Priyanka Jain <priyanka.jain@nxp.com>