mfd: Move TPS55910 Kconfig option

Having another TPS chip at the end of the Kconfig when all it's
relatives are grouped together in their own section seems totally
counter-intuitive. Move it, also in the Makefile.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>