mailbox: Introduce support for T-head TH1520 Mailbox driver

This driver was tested using the drm/imagination GPU driver. It was able
to successfully power on the GPU, by passing a command through mailbox
from E910 core to E902 that's responsible for powering up the GPU. The
GPU driver was able to read the BVNC version from control registers,
which confirms it was successfully powered on.

[   33.957467] powervr ffef400000.gpu: [drm] loaded firmware
powervr/rogue_36.52.104.182_v1.fw
[   33.966008] powervr ffef400000.gpu: [drm] FW version v1.0 (build
6621747 OS)
[   38.978542] powervr ffef400000.gpu: [drm] *ERROR* Firmware failed to
boot

Though the driver still fails to boot the firmware, the mailbox driver
works when used with the not-yet-upstreamed firmware AON driver. There
is ongoing work to get the BXM-4-64 supported with the drm/imagination
driver [1], though it's not completed yet.

This work is based on the driver from the vendor kernel [2].

Link: https://gitlab.freedesktop.org/imagination/linux-firmware/-/issues/2 [1]
Link: https://github.com/revyos/thead-kernel.git [2]

Signed-off-by: Michal Wilczynski <m.wilczynski@samsung.com>
Signed-off-by: Jassi Brar <jassisinghbrar@gmail.com>

MAINTAINERS

@@ -19871,6 +19871,7 @@ T:	git https://github.com/pdp7/linux.git
 F:	Documentation/devicetree/bindings/clock/thead,th1520-clk-ap.yaml
 F:	arch/riscv/boot/dts/thead/
 F:	drivers/clk/thead/clk-th1520-ap.c
+F:	drivers/mailbox/mailbox-th1520.c
 F:	include/dt-bindings/clock/thead,th1520-clk-ap.h
 
 RNBD BLOCK DRIVERS

drivers/mailbox/Kconfig

@@ -296,4 +296,14 @@ config QCOM_IPCC
 	  acts as an interrupt controller for receiving interrupts from clients.
 	  Say Y here if you want to build this driver.
 
+config THEAD_TH1520_MBOX
+	tristate "T-head TH1520 Mailbox"
+	depends on ARCH_THEAD || COMPILE_TEST
+	help
+	  Mailbox driver implementation for the Thead TH-1520 platform. Enables
+	  two cores within the SoC to communicate and coordinate by passing
+	  messages. Could be used to communicate between E910 core, on which the
+	  kernel is running, and E902 core used for power management among other
+	  things.
+
 endif

drivers/mailbox/Makefile

@@ -64,3 +64,5 @@ obj-$(CONFIG_SPRD_MBOX)		+= sprd-mailbox.o
 obj-$(CONFIG_QCOM_CPUCP_MBOX)	+= qcom-cpucp-mbox.o
 
 obj-$(CONFIG_QCOM_IPCC)		+= qcom-ipcc.o
+
+obj-$(CONFIG_THEAD_TH1520_MBOX)	+= mailbox-th1520.o