boards/risc-v/qemu-rv: Add missing section on ld-nuttsbi.script

Fix an issue regarding the `rv-virt:nsbi` defconfig, which wasn't
booting properly because `sbi_set_mtimecmp` failed to get the
mtimecmp's register address when evaluating `g_mtimecmp`. This
variable, which should be located at the `.noinit` section, wasn't
being set accordingly by the linker script, ending up on `bss
section which is zero-initialized, overwriting its value previously
set by `sbi_init_mtimer`.

boards/risc-v/qemu-rv/rv-virt/scripts/ld-nuttsbi.script

@@ -155,6 +155,12 @@ SECTIONS
       _edata = . ;
     }
 
+  .noinit (NOLOAD) : ALIGN(4)
+    {
+      *(.noinit)
+      *(.noinit.*)
+    } > ksram
+
   .bss :
     {
       _sbss = . ;