amd/mendocino: Control DPTC with only Kconfig

amd/mendocino: Control DPTC with only Kconfig

SOC_AMD_COMMON_BLOCK_ACPI_DPTC can be enabled conditionally for any
skyrim boards, similar to mainboard/google/zork/Kconfig. This makes the
value dptc_tablet_mode_enable redundant.

This CL removes dptc_tablet_mode_enable so DPTC is controlled entirely
with the Kconfig value SOC_AMD_COMMON_BLOCK_ACPI_DPTC. This means DPTC
is only included for boards that actually enable it.

BRANCH=none
BUG=b:217911928
TEST=emerge-skyrim coreboot

Signed-off-by: Tim Van Patten <timvp@google.com>
Change-Id: I73fca5a16826313219247f452d37fb526ad4f4df
Reviewed-on: https://review.coreboot.org/c/coreboot/+/67639
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Matt DeVillier <matt.devillier@amd.corp-partner.google.com>
Reviewed-by: Felix Held <felix-coreboot@felixheld.de>