MIPS: ath79: Store the SoC revision in a global variable

Knowing the exact revision of the SoC is required to make runtime decisions
in various code paths.  We have determined the SoC revision already, so we
only need to store that in a global variable.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Cc: Imre Kaloz <kaloz@openwrt.org>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/3027/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>