soc/amd/picasso: correct MCFG ACPI table

The start and end bus number in the MCFG ACPI table is inclusive.
Therefore, the number of buses decoded needs to be subtracted by
1.

BUG=b:158874061

Change-Id: Ic773bc1e0ccaa99af45d1a53919f6480887fa37e
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/42329
Reviewed-by: Furquan Shaikh <furquan@google.com>
Reviewed-by: Raul Rangel <rrangel@chromium.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/soc/amd/picasso/acpi.c

@@ -33,7 +33,7 @@ unsigned long acpi_fill_mcfg(unsigned long current)
 					     CONFIG_MMCONF_BASE_ADDRESS,
 					     0,
 					     0,
-					     CONFIG_MMCONF_BUS_NUMBER);
+					     CONFIG_MMCONF_BUS_NUMBER - 1);
 
 	return current;
 }