realtek: Keep extern declaration in headers

realtek: Keep extern declaration in headers

The common declarations should not be spread around in different source
files but kept inside the header files. This is unfortunately currently not
the best place to store them because soc_info is actually from non-DSA
code. But it is at least better than having them in diffent source files.

Signed-off-by: Sven Eckelmann <[email protected]>
Link: https://github.com/openwrt/openwrt/pull/20906
Signed-off-by: Hauke Mehrtens <[email protected]>