board: freescale: ls2080ardb: Update QIXIS code

Update QIXIS related code to be executed only if CONFIG_FSL_QIXIS
flag is enabled. In case QIXIS code is not enabled, use default
sysclk value as 100MHz per board documentation.

Signed-off-by: Priyanka Jain <priyanka.jain@nxp.com>
Reviewed-by: York Sun <york.sun@nxp.com>