clk: at91: make of_sama5d2_clk_generated_setup() static

The of_sama5d2_clk_generated_setup() is not exported outside
of the driver, so make it static to fix the warning about it
being not static:

drivers/clk/at91/clk-generated.c:270:13: warning: symbol 'of_sama5d2_clk_generated_setup' was not declared. Should it be static?

Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Acked-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>