clk: mediatek: mt7981-topckgen: flag SGM_REG_SEL as critical

Without the SGM_REG_SEL clock enabled the cpu freezes if trying to
access registers used by MT7981 clock drivers itself.
Mark SGM_REG_SEL as critical to make sure it is always enabled to
prevent freezes on boot even if the Ethernet driver which prepares
and enables the clock is not loaded or probed at a later point.

Fixes: 813c3b53

 ("clk: mediatek: add MT7981 clock support")
Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Link: https://lore.kernel.org/r/fc157139e6b7f8dfb6430ac7191ba754027705e8.1708221995.git.daniel@makrotopia.org


Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>