soc: silabs: Remove counter dependency for pm

Power Manager no longer requires the Counter driver. This seems to have
been a hack to get the Sleeptimer HAL included in the build, as the
Sleeptimer is the real dependency of the Silabs Power Manager HAL.

Since Sleeptimer is now used for the OS timer, this hack is not needed.

Signed-off-by: Aksel Skauge Mellbye <aksel.mellbye@silabs.com>