reset: socfpga: declare socfpga_reset_init in a header file

Avoid declaring extern functions in c files. To make sure function
definition and usage don't get out of sync, declare socfpga_reset_init
in a common header.

Suggested-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Acked-by: Dinh Nguyen <dinguyen@kernel.org>