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