boards: shields: Clean up Kconfig.defconfig files

Shield's Kconfig.defconfig files should only be used to configure
components options, or drivers specific (eg init priority, ..), depending
on driver activation which remains application responsibility.
High level components (such as DISPLAY) should not be enabled by default on
boards activation (which is similar to policy used for boards).

Additionally, clean up busses activation (SPI, I2C), which is not required
as bus activation is already done automatically on bus devices activation.

Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>