lib: Add rp2040 build files

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

lib/rp2040/hardware/structs/pll.h

@@ -0,0 +1,25 @@
+/*
+ * Copyright (c) 2020 Raspberry Pi (Trading) Ltd.
+ *
+ * SPDX-License-Identifier: BSD-3-Clause
+ */
+
+#ifndef _HARDWARE_STRUCTS_PLL_H
+#define _HARDWARE_STRUCTS_PLL_H
+
+#include "hardware/address_mapped.h"
+#include "hardware/regs/pll.h"
+
+/// \tag::pll_hw[]
+typedef struct {
+    io_rw_32 cs;
+    io_rw_32 pwr;
+    io_rw_32 fbdiv_int;
+    io_rw_32 prim;
+} pll_hw_t;
+
+#define pll_sys_hw ((pll_hw_t *const)PLL_SYS_BASE)
+#define pll_usb_hw ((pll_hw_t *const)PLL_USB_BASE)
+/// \end::pll_hw[]
+
+#endif