Integrate uACPI & remove own ACPI parsing code

This commit adds uACPI as a submodule and removes all handwritten table
parsing code as well as struct definitions, since they now work via
uACPI. There's also now an ability to shut down by using the power
button.

kernel-src/include/kernel/acpi.h

@@ -0,0 +1,13 @@
+#ifndef _ACPI_H
+#define _ACPI_H
+
+// Doesn't depend on PCI, only makes table search available
+void acpi_early_init(void);
+
+// Loads the actual ACPI namespace, hooks up the power button
+void acpi_init(void);
+
+int acpi_poweroff(void);
+int acpi_reboot(void);
+
+#endif