add device platform module with ACPI support

src/root.zig

@@ -95,4 +95,9 @@ pub const BootInfo = extern struct {
     /// The kernel's own PT_LOAD segments (it has three: text, rodata, data).
     kernel_segments: [8]KernelSegment,
     kernel_segment_count: u32,
+    /// Physical address of the ACPI RSDP the firmware exposed, or 0 if none. The
+    /// kernel's device layer parses the ACPI tables from here to discover hardware.
+    /// A device-tree boot path leaves this 0 and (later) fills a `device_tree_blob`
+    /// field instead, so the kernel discovers devices without knowing what booted it.
+    acpi_rsdp: u64 = 0,
 };