ARC: boards: HSDK: ged rid of pinmux usage

Pinmux is deprecated (see #39740) so let's get rid of
it's usage for HSDK board.

As we call pinmux only once at init phase we simply do
register setup in platform code instead of pinmux.

Signed-off-by: Evgeniy Paltsev <PaltsevEvgeniy@gmail.com>
Signed-off-by: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>