ANDROID: trusty-core: use dev_groups for trusty_version attribute

Setting device_driver::dev_groups is the recommended way to register
device attributes.

Note: this changes the behavior slightly, as we now register the
trusty_version attribute even if requesting the version from Trusty
failed.  The file will show "unknown" in that case.  (Don't return an
error instead, since returning an error when reading a sysfs attribute
would be unusual.)

Change-Id: Idc2fe1bd50e428735e0e2ba7e5e9aee4b93474f0
Signed-off-by: Eric Biggers <ebiggers@google.com>