imx6: cache: disable L2 before touching Auxiliary Control Register

imx6: cache: disable L2 before touching Auxiliary Control Register

According PL310 TRM, Auxiliary Control Register
"
The register must be written to using a secure access, and it can be
read using either a secure or a NS access. If you write to this register
with a NS access, it results in a write response with a DECERR response,
and the register is not updated. Writing to this register with the L2
cache enabled, that is, bit[0] of L2 Control Register set to 1,
results in a SLVERR.
"

So If L2 cache is already enabled by ROM, chaning value of ACR
will cause SLVERR and uboot hang.

Signed-off-by: Peng Fan <[email protected]>
Cc: Stefano Babic <[email protected]>
Cc: Fabio Estevam <[email protected]>