PCI: mediatek-gen3: Add comment about initialization order in mtk_pcie_en7581_power_up()

PCI: mediatek-gen3: Add comment about initialization order in mtk_pcie_en7581_power_up()

Add a comment in mtk_pcie_en7581_power_up() to clarify, unlike the other
MediaTek Gen3 controllers, the Airoha EN7581 requires PHY initialization
and power-on before PHY reset deassert.

Link: https://lore.kernel.org/r/20250108-pcie-en7581-fixes-v6-3-21ac939a3b9b@kernel.org
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Krzysztof Wilczyński <kwilczynski@kernel.org>
Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>