pwm: Kconfig: Enable pwm-tiecap to be built for Keystone

66AK2G SoC has ECAP subsystem that is used as pwm-backlight provider for
display. Hence, enable pwm-tiecap driver to be built for Keystone
architecture.

Signed-off-by: Vignesh R <vigneshr@ti.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>