clk: at91: main: warn when the main crystal frequency is not set

When the main crystal frequency is not set, the main clock is approximated using
the MAINF value in the CKGR_MCFR register. Warn the user in that case.

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Acked-by: Boris BREZILLON <boris.brezillon@overkiz.com>
Signed-off-by: Mike Turquette <mturquette@linaro.org>