platform/lib: Allow the OS to map the regions that are protected by PMP

This is achieved by removing the 'no-map' property from the
'reserved-memory' node when PMP is present, otherwise we keep it as it
offers a small protection if the OS does not map this region at all.
A new callback in platform_override is introduced and allows to fixup the
device-tree. It is used here to override this new default behaviour on
SiFive Fu540 platforms that has an erratum that prevents S-mode software
to access a PMP protected region using 1GB page table mapping.

If PMP is present, telling the OS not to map the reserved regions does not
add much protection since it only avoids access to regions that are already
protected by PMP. But by not allowing the OS to map those regions, it
creates holes in the OS system memory map and prevents the use of
hugepages which would generate, among other benefits, less TLB miss.

Signed-off-by: Alexandre Ghiti <alex@ghiti.fr>
Reviewed-by: Atish Patra <atish.patra@wdc.com>
Reviewed-by: Anup Patel <anup.patel@wdc.com>

include/sbi_utils/fdt/fdt_fixup.h

@@ -47,6 +47,20 @@ void fdt_plic_fixup(void *fdt, const char *compat);
  */
 int fdt_reserved_memory_fixup(void *fdt);
 
+/**
+ * Fix up the reserved memory subnodes in the device tree
+ *
+ * This routine adds the no-map property to the reserved memory subnodes so
+ * that the OS does not map those PMP protected memory regions.
+ *
+ * Platform codes must call this helper in their final_init() after fdt_fixups()
+ * if the OS should not map the PMP protected reserved regions.
+ *
+ * @param fdt: device tree blob
+ * @return zero on success and -ve on failure
+ */
+int fdt_reserved_memory_nomap_fixup(void *fdt);
+
 /**
  * General device tree fix-up
  *