mips: octeon: use mips_mach_early_init() to copy to L2 cache

mips: octeon: use mips_mach_early_init() to copy to L2 cache

This patch adds the code to copy itself from bootrom location to a
different location (TEXT_BASE) to the Octeon platform. Its used in
this case to copy the complete U-Boot image into L2 cache, which
greatly improves the bootup time - especially in regard to the
very long and complex DDR4 init code.

The Kconfig symbol CONFIG_MIPS_MACH_EARLY_INIT is enabled with this
patch for Octeon.

Signed-off-by: Stefan Roese <sr@denx.de>