mfd: tps65218: Correct the config description

When the initial support was added for this PMIC was added
only regulator support was present. Now we have GPIO and Powerbutton
support as well. Hence correct the description of MFD_TPS65218 config
option.

Signed-off-by: Keerthy <j-keerthy@ti.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>