soc/amd/picasso: add Kconfig option to disable rom sharing

Add a knob for mainboards to request disablement of the SPI
flash ROM sharing in the chipset. The chipset allows the board
to share the SPI flash bus and needs a pin to perform the request.
If the board design does not employ SPI flash ROM sharing then it's
imperative to ensure this option is selected, especially if the
pin is being utilized by something else in the board design.

BUG=b:153502861

Change-Id: I60ba852070dd218c4ac071b6c1cfcde2df8e5dce
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/coreboot/+/2146445
Reviewed-by: Furquan Shaikh <furquan@chromium.org>
Commit-Queue: Aaron Durbin <adurbin@google.com>
Tested-by: Aaron Durbin <adurbin@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/40869
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Furquan Shaikh <furquan@google.com>

src/soc/amd/picasso/southbridge.c

@@ -360,6 +360,9 @@ void fch_early_init(void)
 {
 	sb_print_pmxc0_status();
 	i2c_soc_early_init();
+
+	if (CONFIG(DISABLE_SPI_FLASH_ROM_SHARING))
+		lpc_disable_spi_rom_sharing();
 }
 
 void sb_enable(struct device *dev)