machine-specific-hooks.inc: limit am57xx-evm device tree to am57xx-beagle-x15
machine-specific-hooks.inc: limit am57xx-evm device tree to am57xx-beagle-x15

am57xx-beagle-x15.dts is available across all the kernels that we care
(4.4 LTS, 4.9 LTS, mainline, etc...). Since revb1 isn't available in 4.4,
limit am57xx-evm device tree to am57xx-beagle-x15.

Signed-off-by: Fathi Boudra <7cd6e55cc07a66bc71d7901a4cddfe6c22aab659@linaro.org>

kraj/meta-96boards,fboudra/meta-96boards,mrchapp/meta-96boards,96boards/meta-96boards,kraj/meta-96boards