arch/arm32: restore the 32-bit Raspberry Pi 4 platform

[daniel-rossier]

Why

The 32-bit rpi4 platform was dropped when the tree moved to rpi4_64 — arch/arm32/Kconfig now only carries VIRT32. But the tree is still written for it:

kernel/Kconfig:48          depends on SMP && (RPI4 || RPI4_64)
devices/serial/Kconfig:12  depends on RPI4 || RPI4_64
devices/i2c/Kconfig:5      depends on RPI4 || RPI4_64     (e09572557)
devices/rpisense/Kconfig:3 depends on RPI4 || RPI4_64     (e09572557)

Those || RPI4 clauses have been pointing at a symbol that does not exist. Only the arch symbol and its wiring were missing.

What

• config RPI4 in arch/arm32/Kconfig (select GIC_V2, as VIRT32 and RPI4_64 do)
• TARGET = rpi4 in the top Makefile, dtb-$(CONFIG_RPI4) in dts/Makefile
• arch/arm32/rpi4/ — Makefile, platsmp.c, mach/io.h. Unlike arm64, which releases the secondaries through the spin table, the 32-bit path wakes them by writing the entry point into mailbox 3 of the ARM local interrupt controller — hence LOCAL_INTC_PHYS, which the arm64 mach/io.h does not need.
• dts/rpi4.dts and configs/rpi4_defconfig, mirroring their rpi4_64 counterparts with the 32-bit values:

	rpi4 (arm32)	rpi4_64 (arm64)
address/size cells	1	2
KERNEL_VADDR	0xc0000000	0xffff800000000000
IO_MAPPING_BASE	0xe0000000	0xffff900000000000
memory base	0x01000000	0x01000000
FIT load / text offset	0x01008000 / 0x8000	0x01080000 / 0x80000

Notes

• The mini UART node carries its AUX interrupt (SPI 93, the value Broadcom's bcm2711-rpi-4-b.dtb gives for /soc/serial@7e215040). bcm283x_mu.c never reads "interrupts" today, so it is inert — it is simply what the hardware has, and it lines the node up with what rpi4: make Ctrl-C work on the mini UART console #304 does for rpi4_64.
• i2c1 and rpisense are status = "disabled" and CONFIG_I2C is off, exactly as in the rpi4_64 pair — enabling them is a board-specific choice, since they are CORE drivers and would probe a Sense HAT that may not be fitted.
• U-Boot needs nothing: rpi_4_32b_defconfig and the shared uEnv bootcmd in include/configs/rpi.h were already in the patch set, left over from when the platform existed.

Test

Booted on a real RPi 4 Model B: U-Boot 2022.04 (armv7) loads rpi4.itb and SO3 reaches user space. The 64-bit chain is unaffected — both were rebuilt and booted from the same card.