ARM: dts: am437x: Correct DWC USB3 compatible string

Syonpsys IP cores are supposed to be defined with "snps" vendor-prefix.
Use it instead of the deprecated "synopsys" one.

Signed-off-by: Serge Semin <Sergey.Semin@baikalelectronics.ru>
Reviewed-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Tony Lindgren <tony@atomide.com>