imx: mx6ull: skip setting ahb clock

Rom already initialized clock at 396M and 132M for arm core and ahb,
so skip setting them again in U-Boot.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Cc: Stefano Babic <sbabic@denx.de>
Reviewed-by: Stefano Babic <sbabic@denx.de>