ANDROID: GKI: Add rockpi4 fragment and build.config

Add support for the Rock Pi 4 board downstream of GKI. This removes a
bitrotting custom defconfig and replaces it with a fragment based on
the GKI. Add the build.config for build automation purposes.

This configuration is not yet GKI compatible. Some options must be set
that are not GKI compliant:

CONFIG_ARCH_ROCKCHIP=y
CONFIG_COMMON_CLK_ROCKCHIP=y
CONFIG_DEVTMPFS=y
CONFIG_ROCKCHIP_GRF=y
CONFIG_ROCKCHIP_IOMMU=y
CONFIG_ROCKCHIP_MBOX=y
CONFIG_ROCKCHIP_PM_DOMAINS=y
CONFIG_ROCKCHIP_TIMER=y

Bug: 183963484
Change-Id: Id7376d6d377f453a3fe2c215d10d0424550de1bc
Signed-off-by: Alistair Delva <adelva@google.com>
Signed-off-by: Matthias Maennich <maennich@google.com>