firmware: dell_rbu: Make payload memory uncachable

The dell_rbu driver takes firmware update payloads and puts them in memory so
the system BIOS can find them after a reboot.  This sometimes fails (though
rarely), because the memory containing the payload is in the CPU cache but
never gets written back to main memory before the system is rebooted (CPU
cache contents are lost on reboot).

With this patch, the payload memory will be changed to uncachable to ensure
that the payload is actually in main memory before the system is rebooted.

Signed-off-by: Stuart Hayes <stuart.w.hayes@gmail.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>