armv8: fsl-layerscape: fix compile warning "rcw_tmp"

arch/arm/cpu/armv8/fsl-layerscape/fsl_lsch2_speed.c: In function
‘get_sys_info’:
arch/arm/cpu/armv8/fsl-layerscape/fsl_lsch2_speed.c:29:6: warning:
unused variable ‘rcw_tmp’ [-Wunused-variable]
  u32 rcw_tmp;

Signed-off-by: Prabhakar Kushwaha <prabhakar.kushwaha@nxp.com>
Reviewed-by: York Sun <york.sun@nxp.com>