kernel: laptop touchpads over I2C-HID

A laptop's touchpad is on the chipset's I2C bus, found only through ACPI
(PNP0C50 / ACPI0C50), and SchOS had no I2C at all, so the touchpad of
every recent laptop did nothing.

kernel/acpi/i2c_hid.c drives the DesignWare I2C controller every x86
chipset uses (AMDI0010 on Ryzen) by polling, speaks HID over I2C to the
device the _DSM points at, and feeds the reports through the USB HID
parser. On AMD the interrupt pin's level is read every tick, so a report
is read exactly when one is waiting; elsewhere the input register is polled.

The touchpad is driven in mouse mode, which it starts in after a reset:
motion, clicks and taps become /dev/input/event1 like any mouse. The HID
parser now prefers a descriptor's relative pointer over an absolute one
for this, and takes descriptors from transports other than USB.

Untested on hardware: QEMU emulates no I2C touchpad.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01S1giNvrHTrPMPw7V6x1kQ4

Makefile

@@ -101,7 +101,7 @@ KERN_ACPI = $(addprefix $(BUILD)/uacpi/, \
               namespace.o notify.o opcodes.o opregion.o osi.o registers.o \
               resources.o shareable.o sleep.o stdlib.o tables.o types.o \
               uacpi.o utilities.o)
-KERN_ACPI += $(addprefix $(BUILD)/kernel/acpi/, host.o aml.o)
+KERN_ACPI += $(addprefix $(BUILD)/kernel/acpi/, host.o aml.o i2c_hid.o)
 
 # zstd's decoder, for SCFF -- the compressed live root (include/fs/scff.h).
 # Unpacked by ports/zstd into build/ports/zstd/src and compiled into the kernel