hw/misc/imx6ul_ccm: Implement non writable bits in CCM registers

hw/misc/imx6ul_ccm: Implement non writable bits in CCM registers

Some bits of the CCM registers are non writable.

This was left undone in the initial commit (all bits of registers were
writable).

This patch adds the required code to protect the non writable bits.

Signed-off-by: Jean-Christophe Dubois <jcd@tribudubois.net>
Message-id: 20200608133508.550046-1-jcd@tribudubois.net
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>