clk: rockchip: add sclk_mac_lbtest to rk3188_critical_clocks

Since the loopbacktest clock is not exported and is not touched in the
driver, it has to be added to rk3188_critical_clocks to be protected from
being disabled and in order to get the emac working.

Signed-off-by: Alex Bee <knaerzche@gmail.com>
Link: https://lore.kernel.org/r/20200722161820.5316-1-knaerzche@gmail.com


Signed-off-by: Heiko Stuebner <heiko@sntech.de>