regulator: tps65910: Add missing #include <linux/of.h>

drivers/regulator/tps65910-regulator.c: In function ‘tps65910_parse_dt_reg_data’:
drivers/regulator/tps65910-regulator.c:1018: error: implicit declaration of function ‘of_get_child_by_name’
drivers/regulator/tps65910-regulator.c:1018: warning: assignment makes pointer from integer without a cast
drivers/regulator/tps65910-regulator.c:1034: error: implicit declaration of function ‘of_node_put’
drivers/regulator/tps65910-regulator.c:1056: error: implicit declaration of function ‘of_property_read_u32’

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Mark Brown <broonie@kernel.org>