clk: at91: keep slow clk enabled to prevent system hang

All slow clk users are not properly claiming it (get + prepare + enable)
before using it.
If all users properly claiming this clock release it, the clock is
disabled, but faulty users still depends on it, and the system hangs.

This fix prevents the slow clock from being disabled, and should solve the
hanging issue, but offending drivers should be patched to properly claim
this clock.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Reported-by: Bo Shen <voice.shen@atmel.com>
Cc: stable@vger.kernel.org
Signed-off-by: Michael Turquette <mturquette@linaro.org>