mfd: madera: Fixup SPDX headers

GPL-2.0-only is the preferred way of expressing v2 of the GPL, so switch
to that. Remove some redundant copyright notices and correct some
instances where the wrong comment type has been used in header files.

Signed-off-by: Charles Keepax <ckeepax@opensource.cirrus.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>