arm64: dts: synaptics: remove unused DTSI for AS370

The as370.dtsi for Synaptics AS370 SoC does not have a user (DTS board
file), is uncompilable and untestable.  It was added back in 2018.  No
user appeared since that time, so assume it won't be added.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>