mx5: Remove CONFIG_L2_OFF and CONFIG_SYS_L2CACHE_OFF

mx5: Remove CONFIG_L2_OFF and CONFIG_SYS_L2CACHE_OFF

CONFIG_L2_OFF is obsolete after the following commit:

e47f2db5371047eb9bcd115fee084e6a8a92a239
armv7: rename cache related CONFIG flags
Replace the cache related CONFIG flags with more meaningful
names. Following are the changes:
CONFIG_L2_OFF	     -> CONFIG_SYS_L2CACHE_OFF

Since imx5 does not provide L2 cache operations(Enable/Disable)
Simply remove CONFIG_L2_OFF and CONFIG_SYS_L2CACHE_OFF

Signed-off-by: Jason Liu <jason.hui@linaro.org>
Cc:Stefano Babic <sbabic@denx.de>
Acked-by: Stefano Babic <sbabic@denx.de>