mfd: syscon: Support native-endian regmaps

The regmap devicetree binding documentation states that a native-endian
property should be supported as well as big-endian & little-endian,
however syscon in its duplication of the parsing of these properties
omits support for native-endian. Fix this by setting
REGMAP_ENDIAN_NATIVE when a native-endian property is found.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: Lee Jones <lee.jones@linaro.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Guenter Roeck <linux@roeck-us.net>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: linux-mips@linux-mips.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>