ramips: improve MT7621S core detection

ramips: improve MT7621S core detection

The proposed detection method was based on reading the LAUNCH_FREADY core flag.
However, this method only works before the cores are launched.
For this reason, the core number detection method has been changed to a simpler one.
For mt6721s the 17th revision bit is zero, hence we know that it is this chip,
so the number of cores is 1.

Fixes: https://github.com/openwrt/openwrt/issues/17764
Tested-by: Enrico Mioso <[email protected]>
Tested-by: Simon Etzlstorfer <[email protected]>
Tested-by: Mauri Sandberg <[email protected]>
Co-authored-by: Shiji Yang <[email protected]>
Signed-off-by: Mieczyslaw Nalewaj <[email protected]>
Link: https://github.com/openwrt/openwrt/pull/17834
Signed-off-by: Hauke Mehrtens <[email protected]>