Fix compilation for u540

kernel/src/arch/riscv32/board/u540/mod.rs

@@ -1,3 +1,6 @@
+#[path = "../../../../drivers/gpu/fb.rs"]
+pub mod fb;
+
 use super::consts::KERNEL_OFFSET;
 use crate::memory::phys_to_virt;
 
@@ -24,3 +27,7 @@ pub unsafe fn enable_serial_interrupt() {
     const UART_RXWM: u8 = 0x2;
     SERIAL_BASE.add(UART_REG_IE).write_volatile(UART_RXWM);
 }
+
+pub fn probe_fb_info(_width: u32, _height: u32, _depth: u32) -> fb::FramebufferResult {
+    unimplemented!()
+}