drivers/pc80: Add optional spinlock for nvram CBFS access

drivers/pc80: Add optional spinlock for nvram CBFS access

When enabling the IOMMU on certain systems dmesg is spammed with I/O page faults like the following:
AMD-Vi: Event logged [IO_PAGE_FAULT device=00:14.0 domain=0x000a address=0x000000fdf9103300 flags=0x0030]

Decoding the faulting address:
0x000000fdf9103300
        fdf91x          Hypertransport system management region
              33        SysMgtCmd (System Management Command) = 0x33
              3         Base Command Type = 0x3: STPCLK (Stop Clock request)
               3        SMAF (System Management Action Field) = [3:1] = 0x1
               1        Signal State Bit Map = [0] = 0x1

Therefore, the error appears to be triggered by an upstream C1E request.

This was eventually traced to concurrent access to the SP5100's SPI Flash controller by
multiple APs during startup.  Calls to the nvram read functions get_option and read_option
call CBFS functions, which in turn make near-simultaneous requests to the SPI Flash
controller, thus placing the SP5100 in an invalid state.  This limitation is not documented
in any public AMD errata, and was only discovered through considerable debugging effort.

Change-Id: I4e61b1ab767b1b7958ac7c1cf20eee41d2261bef
Signed-off-by: Timothy Pearson <tpearson@raptorengineeringinc.com>
Reviewed-on: https://review.coreboot.org/12061
Tested-by: build bot (Jenkins)
Reviewed-by: Martin Roth <martinroth@google.com>