Now that SSDK has been updated to use in-kernel SMEM ID-s to identify
the SoC its running on instead of relying on the downstream socinfo.h
header we can move the read_ipq_soc_version_major() function directly to
cpr3-util.c as its the only user of anything from the header and drop it.
Link: https://github.com/openwrt/openwrt/pull/15786
Signed-off-by: Robert Marko <robimarko@gmail.com>