clk: mvebu: flag the crypto clk as CLK_IGNORE_UNUSED

The crypto SRAM, used by the armada 370 cpuidle code to workaround a bug
in the BootROM code, requires the crypto clk to be up and running.
Flag the crypto clk as IGNORE_UNUSED until we add the proper
infrastructure to define the crypto SRAM in the DT and reference the crypto
clk in this SRAM node.

Reported-by: Kevin Hilman <khilman@linaro.org>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Tested-by: Kevin Hilman <khilman@linaro.org>
Signed-off-by: Michael Turquette <mturquette@baylibre.com>