package/ti-gfx: list supported SoCs in Config.in help text

package/ti-gfx: list supported SoCs in Config.in help text

The ti-gfx package supports a set of SGX implementations which are
used in some TI SoCs, newer parts are not supported by this package.
This patch adds a list of supported TI SoCs to the help text.

Signed-off-by: Lothar Felten <lothar.felten@gmail.com>
[Thomas: update with the new name of packages: ti-sgx-{km,um}.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>