i2c: piix4, x86/platform: Move the SB800 PIIX4 FCH definitions to <asm/amd/fch.h>

SB800_PIIX4_FCH_PM_ADDR is used to indicate the base address for the
FCH PM registers.  Multiple drivers may need this base address, so
move related defines to a common header location and rename them
accordingly.

Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Andi Shyti <andi.shyti@kernel.org>
Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Hans de Goede <hdegoede@redhat.com>
Cc: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Cc: Jean Delvare <jdelvare@suse.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Sanket Goswami <Sanket.Goswami@amd.com>
Cc: Shyam Sundar S K <Shyam-sundar.S-k@amd.com>
Cc: Yazen Ghannam <yazen.ghannam@amd.com>
Cc: linux-i2c@vger.kernel.org
Link: https://lore.kernel.org/r/20250422234830.2840784-4-superm1@kernel.org