arch: rp2040: Fix the initial stack pointer

Summary:
- The NuttX for raspberrypi-pico boots via the boot_stage2 provided
  by the pico-sdk which sets the MSP at the end of the SRAM.
- However, the NuttX expects the MSP is set to the top of the idle stack.
- This commit fixes this issue.

Impact:
- None

Testing:
- Tested with nsh, nshsram and smp configrations

arch/arm/src/rp2040/rp2040_start.c

@@ -89,6 +89,10 @@ void __start(void)
   uint32_t *dest;
   int i;
 
+  /* Set MSP to the top of the IDLE stack */
+
+  __asm__ __volatile__ ("\tmsr msp, %0\n" :: "r" (g_idle_topstack));
+
   if (up_cpu_index() != 0)
     {
       while (1)