Enabling support for more than one BMIPS CPU in the same build may
result in different L1_CACHE_SHIFT values, e.g.

    CPU_BMIPS5000 selects MIPS_L1_CACHE_SHIFT_7
    CPU_BMIPS4380 selects MIPS_L1_CACHE_SHIFT_6
    anything else defaults to MIPS_L1_CACHE_SHIFT_5

Ensure that if more than one MIPS_L1_CACHE_SHIFT_x option is selected,
Kconfig sets CONFIG_MIPS_L1_CACHE_SHIFT to the highest value.
Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
Cc: f.fainelli@gmail.com
Cc: jaedon.shin@gmail.com
Cc: abrestic@chromium.org
Cc: tglx@linutronix.de
Cc: jason@lakedaemon.net
Cc: jogo@openwrt.org
Cc: arnd@arndb.de
Cc: computersforpeace@gmail.com
Cc: linux-mips@linux-mips.org
Cc: devicetree@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/8847/Signed-off-by: Ralf Baechle <ralf@linux-mips.org>