phy: rockchip-snps-pcie3: only look for rockchip,pipe-grf on rk3588

phy: rockchip-snps-pcie3: only look for rockchip,pipe-grf on rk3588

The rockchip,pipe-grf property is only used on rk3588, but not on
rk3568. Therefore this property is not present on rk3568 devices,
leading to the following message:

  rockchip-snps-pcie3-phy fe8c0000.phy: failed to find rockchip,pipe_grf regmap

Fix that by only looking for this property on rk3588.

Fixes: 2e9bffc4f713d ("phy: rockchip: Support PCIe v3")
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
Link: https://lore.kernel.org/r/20220927051752.53089-1-aurelien@aurel32.net
Signed-off-by: Vinod Koul <vkoul@kernel.org>