arm64: dts: socfpga: change the reset-name of "stmmaceth-ocp" to "ahb"

The "stmmaceth-ocp" reset line on the SoCFPGA stmmac ethernet driver is
the same as the "ahb" reset on a standard stmmac ethernet.

commit ("843f6037 dt-bindings: net: snps,dwmac: Add 'ahb'
reset/reset-name") documented the second reset signal as 'ahb' instead
of 'stmmaceth-ocp'. Change the reset-names of the SoCFPGA DWMAC driver to
'ahb'. In order not to break ABI, we will keep support in thedwmac-socfpga
driver to still make use of "stmmaceth-ocp".

This also fixes the dtbs_check warning:
ethernet@ff802000: reset-names:1: 'ahb' was expected

Signed-off-by: Dinh Nguyen <dinguyen@kernel.org>
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v2: update commit message to further describe the reason for the change