dt-bindings: net: snps,dwmac: Add 'ahb' reset/reset-name

According to:
stmmac_platform.c: stmmac_probe_config_dt
stmmac_main.c: stmmac_dvr_probe

dwmac controller may require one (stmmaceth) or two (stmmaceth+ahb)
reset signals, and the maxItems of resets/reset-names is going to be 2.

The gmac of Starfive Jh7110 SOC must have two resets.
it uses snps,dwmac-5.20 IP.

Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Samin Guo <samin.guo@starfivetech.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>