MIPS: Kconfig: Rename several firmware related config symbols.

With the upcoming merge of the ARC architecture there is a small likelyhood
of conflicting use for the CONFIG_ARC config symbol.  Rename it to
CONFIG_FW_ARC.  Also rename CONFIG_ARC32 to CONFIG_FW_ARC32, CONFIG_ARC64
to CONFIG_FW_ARC64.

For consistence also rename CONFIG_SNIPROM to CONFIG_FW_SNIPROM and
CONFIG_CFE to CONFIG_FW_CFE.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>