drivers: entropy: enable ENTROPY_GENERATOR if "zephyr,entropy" is set in DT

Automatically enable ENTROPY_GENERATOR if the device-tree has
any "zephyr,entropy" chosen property specified. This helps
in having CONFIG_ENTROPY_HAS_DRIVER set if the platform support
an entropy driver and this then enables CONFIG_CSPRNG_ENABLED.

Signed-off-by: Valerio Setti <vsetti@baylibre.com>