mimxrt1050: check if D-cache is enabled before enabling it.

An issue has been confirmed in CMSIS core header file in
SCB_EnableDCache when stack is in cacheable memory.

Issue report: https://github.com/ARM-software/CMSIS_5/issues/331



To workaround this issue by checking if Dcache's been enabled before
trying to enable it.

Signed-off-by: Ryan QIAN <jianghao.qian@nxp.com>