mips: add config options for generic cache setup code

mips: add config options for generic cache setup code

Add an own Kconfig symbol for the initial disabling of caches
invoked from generic start code.

Also add an own Kconfig symbols for the initialization of caches
invoked from generic start code.

Until now both code paths could only be disabled with
CONFIG_SKIP_LOWLEVEL_INIT. But this is not flexible enough for
RAM boot scenarios like EJTAG or SPL payload or for machines
which don't require cache initialization or which want to
provide their own cache implementation.

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