platform/x86: oxpec: Rename ec group to tt_toggle

platform/x86: oxpec: Rename ec group to tt_toggle

Currently, the EC group is used for the turbo button. However, the next
patch in the series adds support for the LED button in X1 devices, which
is only applicable for X1 devices. Therefore, rename it to prepare for
adding the second group. And make it const while at it.

Reviewed-by: Derek J. Clark <derekjohn.clark@gmail.com>
Reviewed-by: Thomas Weißschuh <linux@weissschuh.net>
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Antheas Kapenekakis <lkml@antheas.dev>
Link: https://lore.kernel.org/r/20250425111821.88746-7-lkml@antheas.dev
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>