cpufreq: enable 1200Mhz clock speed for armada-37xx

cpufreq: enable 1200Mhz clock speed for armada-37xx

This frequency was disabled because of stability problems whose source could
not be accurately identified[1]. After seven months of testing, the evidence
points to an incorrectly configured bootloader as the source of the historical
instability. Testing was performed on two A3720 devices with this frequency
enabled and the ondemand policy in use. Marvell merged[2] changes to their
bootloader source needed to address the stability issue. This driver should
expose this frequency option to users.

[1] https://github.com/torvalds/linux/commit/484f2b7c61b9ae58cc00c5127bcbcd9177af8dfe
[2] https://github.com/MarvellEmbeddedProcessors/mv-ddr-marvell/pull/44

Signed-off-by: Benjamin Schneider <ben@bens.haus>
Reviewed-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Acked-by: Gregory CLEMENT <gregory.clement@bootlin.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>