dt-bindings: net: snps,dwmac: modify available values of PBL

PBL can be any of the following values: 1, 2, 4, 8, 16 or 32
according to the datasheet, so modify available values of PBL in
snps,dwmac.yaml.

Signed-off-by: Biao Huang <biao.huang@mediatek.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220324012112.7016-2-biao.huang@mediatek.com