mfd: syscon: Set regmap name to DT node name

This sets the regmap name to the device tree node name. This is useful
for debugging.

Signed-off-by: David Lechner <david@lechnology.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Lee Jones <lee.jones@linaro.org>