MIPS: Introduce CAVIUM_RESERVE32 Kconfig option

This options is used to reserve a shared memory region for user processes
to use for hardware memory buffers. The actual code to support the option
comes in the following patch.

Signed-off-by: Corey Minyard <cminyard@mvista.com>
Signed-off-by: Alexander Sverdlin <alexander.sverdlin@nokia.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>