patches/openwrt: uboot-mvebu: Add patches to dynamically support old DDR3 training on Turris Omnia

Add U-Boot patches to dynamically support old DDR3 training on Turris
Omnia.

Users experiencing DDR3 initialization failures or random crashes of the
operating system due to incorrect DDR3 configuration can select the old
DDR3 training implementation to fix those issues by setting the
environment variable
  env set omnia_ddr3_training old
  env save
Or, alternatively from Linux:
  fw_setenv omnia_ddr3_training old