target-arm: Fix handling of SDCR for 32-bit code

target-arm: Fix handling of SDCR for 32-bit code

Fix two issues with our implementation of the SDCR:
 * it is only present from ARMv8 onwards
 * it does not contain several of the trap bits present in its 64-bit
   counterpart the MDCR_EL3

Put the register description in the right place so that it does not
get enabled for ARMv7 and earlier, and give it a write function so that
we can mask out the bits which should not be allowed to have an effect
if EL3 is 32-bit.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 1455892784-11328-2-git-send-email-peter.maydell@linaro.org
Reviewed-by: Sergey Fedorov <serge.fdrv@gmail.com>
Acked-by: Alistair Francis <alistair.francis@xilinx.com>