powerpc: wii.dts: Expose HW_SRNPROT on this platform

This Hollywood register isn’t properly understood, but can allow or
reject access to the SRAM, which we need to set for RTC usage if it
isn’t previously set correctly beforehand.

See https://wiibrew.org/wiki/Hardware/Hollywood_Registers#HW_SRNPROT



Signed-off-by: Emmanuel Gil Peyrot <linkmauve@linkmauve.fr>
Acked-by: Michael Ellerman <mpe@ellerman.id.au> (powerpc)
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20211215175501.6761-4-linkmauve@linkmauve.fr