riscv64: integrate minimal flattened devicetree reader and switch timer to time CSR

Punch list §25 item 0.3 NOT complete.

- Added `starkernel/fdt.h` and `fdt.c` for minimal read-only devicetree parsing: sufficient for boot-time lookups such as `timebase-frequency`.
- Bootloader now captures the devicetree blob (DTB) from `EFI_DTB_TABLE_GUID` into `BootInfo::dtb`.
- RISC-V timer subsystem now uses the `time` CSR as the primary timestamp source, abandoning the hardcoded `cycle` frequency assumption.
- Timer rate is read from `timebase-frequency` in the DTB when accessible; otherwise, a fallback value is used with a RELATIVE trust level.
- Integrated the SBI TIME extension for one-shot timer deadlines, ensuring re-arming occurs after each tick to avoid missing heartbeats.

Verified: riscv64 builds clean, boots to the ok> prompt with no regression; `riscv64/timer.c` reports accurate frequencies on QEMU's default firmware.
Signed-off-by: Robert Allan James <robert.allan.james@gmail.com>

src/starkernel/boot/uefi_loader.c

@@ -773,6 +773,7 @@ EFI_STATUS EFIAPI efi_main(EFI_HANDLE ImageHandle, EFI_SYSTEM_TABLE *SystemTable
     /* Fill BootInfo fields that do NOT require EBS first */
     g_boot_info.runtime_services = SystemTable->RuntimeServices;
     g_boot_info.acpi_table = NULL;
+    g_boot_info.dtb = NULL;
     g_boot_info.framebuffer.base = NULL;
     g_boot_info.framebuffer.size = 0;
     g_boot_info.framebuffer.width = 0;
@@ -805,6 +806,19 @@ EFI_STATUS EFIAPI efi_main(EFI_HANDLE ImageHandle, EFI_SYSTEM_TABLE *SystemTable
         if (!g_boot_info.acpi_table) {
             g_boot_info.acpi_table = acpi10_table;
         }
+
+        /* Devicetree blob, separate pass because the ACPI loop above breaks
+         * early on a 2.0 hit. Absent on ACPI-only platforms; consumers must
+         * handle NULL. riscv64 needs it for timebase-frequency (item 0.3) and
+         * aarch64 for the GIC bases and timer PPI (item 0.6). */
+        for (UINTN i = 0; i < SystemTable->NumberOfTableEntries; ++i)
+        {
+            if (guid_equals(&config_tables[i].VendorGuid, &EFI_DTB_TABLE_GUID))
+            {
+                g_boot_info.dtb = config_tables[i].VendorTable;
+                break;
+            }
+        }
     }
     debug_checkpoint(SystemTable, 6, L"Boot info collected (ACPI table located)");