thermal/drivers/sun8i: Add SRAM register access code

The Allwinner H616 SoC needs to clear a bit in one register in the SRAM
controller, to report reasonable temperature values. On reset, bit 16 in
register 0x3000000 is set, which leads to the driver reporting
temperatures around 200C. Clearing this bit brings the values down to the
expected range. The BSP code does a one-time write in U-Boot, with a
comment just mentioning the effect on the THS, but offering no further
explanation.

To not rely on firmware to set things up for us, add code that queries
the SRAM controller device via a DT phandle link, then clear just this
single bit.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Acked-by: Vasily Khoruzhick <anarsoul@gmail.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20240219153639.179814-6-andre.przywara@arm.com