ARM: keystone: Drop PM domain support for k2g

K2G will use a different power domain driver than the rest of the
keystone family in order to make use of the TI SCI protocol so prevent
the standard keystone pm_domain code from registering itself in
preparation for a new driver.

Acked-by: Santosh Shilimkar <ssantosh@kernel.org>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>
Signed-off-by: Dave Gerlach <d-gerlach@ti.com>
Signed-off-by: Santosh Shilimkar <ssantosh@kernel.org>