arm: mvebu: turris_omnia: Switch DDR speed to 1333H when reset 9 is selected

Users experiencing random kernel crashes due to new versions of
Marvell's DDR training algorithm can solve the issue by setting DDR
speed to 1333H.

But if kernel crashes, it has to be done in U-Boot, which is impossible
without UART connection.

In order to make it easier for users, use the rescue button mechanism:
when rescue mode 9 is selected (that is when 10 LEDs are ON), U-Boot
will train DDR in 1333H mode and also update EEPROM so that subsequent
boot will use this mode.

User has to use the `eeprom` command in U-Boot or `omnia-eeprom` command
in OS to switch back to 1600K mode.

Signed-off-by: Marek Behún <kabel@kernel.org>
Reviewed-by: Stefan Roese <sr@denx.de>