sysreset: Mark driver probe functions as static

These driver probe functions are not (and should not be) called from
outside the respective driver source files. Therefore, the functions
should be marked static.

Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Signed-off-by: Samuel Holland <samuel@sholland.org>
Reviewed-by: Stefan Roese <sr@denx.de>