aspeed: Implement write-1-{set, clear} for AST2500 strapping

aspeed: Implement write-1-{set, clear} for AST2500 strapping

The AST2500 SoC family changes the runtime behaviour of the hardware
strapping register (SCU70) to write-1-set/write-1-clear, with
write-1-clear implemented on the "read-only" SoC revision register
(SCU7C). For the the AST2400, the hardware strapping is
runtime-configured with read-modify-write semantics.

Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Reviewed-by: Joel Stanley <joel@jms.id.au>
Message-id: 20180709143524.17480-1-andrew@aj.id.au
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>