soc/amd: Refactor DPTC Tablet Mode

soc/amd: Refactor DPTC Tablet Mode

Refactor AMD DPTC tablet mode in preparation for adding low/no battery
DPTC settings.

1. Refactor and simplify acpigen_write_alib_dptc() into the following
   functions:
   - acpigen_write_alib_dptc_default()
   - acpigen_write_alib_dptc_tablet()
2. Add device tree register value dptc_tablet_mode_enable to control
   whether DPTC tablet mode is enabled for a variant.
3. Add dptc.asl to perform the necessary ACPI checking before modifying
   the DPTC settings.

BRANCH=none
BUG=b:217911928
TEST=Build zork
TEST=Build nipperkin
TEST=Boot skyrim

Change-Id: I2518fdd526868c9d5668a6018fd3570392e809c0
Signed-off-by: Tim Van Patten <timvp@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/66994
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Karthik Ramasubramanian <kramasub@google.com>