soc: qcom: socinfo: add missing soc_id sysfs entry

Looks like SoC ID is not exported to sysfs for some reason.
This patch adds it!

This is mostly used by userspace libraries like Snapdragon
Neural Processing Engine (SNPE) SDK for checking supported SoC info.

Fixes: efb448d0

 ("soc: qcom: Add socinfo driver")
Reviewed-by: Stephen Boyd <swboyd@chromium.org>
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Link: https://lore.kernel.org/r/20200319121418.5180-1-srinivas.kandagatla@linaro.org
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>