arm64: defconfig: reenable SM_DISPCC_8250

CONFIG_SM_DISPCC_8250 is not enabled by default, but it is still
necessary for the Qualcomm RB5 board. Reenable it (as it was enabled
before the commit dde8cd78 ("arm64: defconfig: rebuild default
configuration")).

Cc: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Fixes: dde8cd78 ("arm64: defconfig: rebuild default configuration")
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>