board: add support for Banana Pi BPI-R4

Add board support for the BananaPi BPI-R4 router board, based on the
MediaTek MT7988A (Filogic 880) SoC with quad-core Cortex-A73 CPU.

The BPI-R4 is the successor to the BPI-R3 but uses entirely different
silicon: the internal switch runs at 2.5 GbE per port, the two SFP+
uplinks use native USXGMII at up to 10 Gbps, and WiFi is provided by
a MediaTek MT7996E PCIe module supporting tri-band WiFi 7 (2.4/5/6 GHz).

Two board variants, BPI-R4-2g5 and BPI-R4P, replace one SFP+ cage with
a 2.5 GbE RJ45 port (using the MT7988A internal 2.5G PHY); the R4P adds
an optional PoE daughterboard on that port.  Both use the upstream
mt7988a-bananapi-bpi-r4-2g5 device tree.

Interfaces:
- Standard R4:  eth1 (gmac1/sfp2 LAN cage) → sfp2, eth2 (gmac2/sfp1 WAN cage) → sfp1
- R4-2g5/R4P:  eth1 (2.5G PHY) → wan, eth2 (gmac2/sfp1 WAN cage) → sfp1

Factory reset / WPS button (GPIO14, active-low): detected by U-Boot at
boot for factory reset; reused by Linux as KEY_WPS_BUTTON at runtime.

eMMC install note: the MT7988A eMMC (pins 38-49) and SD card (pins 32-37)
use separate physical pads so there is no electrical bus sharing, unlike
some BPi-R3 configurations.  However, a single MMC controller
(mmc@11230000) handles both, so only one is active per boot session.
The eMMC install procedure therefore still requires an intermediate SPI
NAND bootloader step until a Linux-side install path is tested.

SD card build (bpi_r4_sd_boot_defconfig) now uses the new
mt7988a_bpir4_sd U-Boot defconfig with cap-sd-highspeed enabled,
giving 50 MHz SD transfers instead of the previous 25 MHz legacy mode.

Also adds MT7988/MT7996 firmware support to the buildroot linux-firmware
package, for 2.5 GbE port PHY and WiFi7 extension board, and updates the
aarch64_defconfig, aarch64_minimal_defconfig, the arch README, CI
boot/image workflows, mkimage.sh, and the ChangeLog.

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

utils/mkimage.sh

@@ -186,6 +186,13 @@ get_bootloader_name()
                 echo "bpi_r3_sd_boot"
             fi
             ;;
+        bananapi-bpi-r4)
+            if [ "$target" = "emmc" ]; then
+                echo "bpi_r4_emmc_boot"
+            else
+                echo "bpi_r4_sd_boot"
+            fi
+            ;;
         bananapi-bpi-r64)
             if [ "$target" = "emmc" ]; then
                 echo "bpi_r64_emmc_boot"