mips: Add CONFIG_MIPS_MACH_EARLY_INIT for very early mach init code

mips: Add CONFIG_MIPS_MACH_EARLY_INIT for very early mach init code

This patch adds the optional call to mips_mach_early_init() to start.S
at a very early stage. Its disabled per default. It can be used for
very early machine / platform specific init code.  Its called very
early and at this stage the PC is allowed to differ from the linking
address (CONFIG_TEXT_BASE) as no absolute jump has been performed until
this call.

It will be used by thje Octeon platform.

Signed-off-by: Stefan Roese <sr@denx.de>
Reviewed-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>