arm64: defconfig: update config for Rockchip PCIe

Now Rockchip PCIe drivers could support both of RC mode and EP
mode, so we need to rename the config name. This patch updates
defconfig to reflect the fact that we want to build Rockchip PCIe
controller as RC mode, into a module as before.

Cc: linux-arm-kernel@lists.infradead.org
Cc: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>