Revert "Documentation/features/vm: Remove arch support status file for 'pte_special'"

The removal of this file appears to have been premature; it's not a feature
enabled by Kconfig, but it's a arch-level feature regardless.  Put it back
for now until some happy future time when we decide how we really want to
document such features.

This reverts commit 2bef69a3.
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/features/vm/pte_special/arch-support.txt

+#
+# Feature name:          pte_special
+#         Kconfig:       __HAVE_ARCH_PTE_SPECIAL
+#         description:   arch supports the pte_special()/pte_mkspecial() VM APIs
+#
+    -----------------------
+    |         arch |status|
+    -----------------------
+    |       alpha: | TODO |
+    |         arc: |  ok  |
+    |         arm: |  ok  |
+    |       arm64: |  ok  |
+    |         c6x: | TODO |
+    |       h8300: | TODO |
+    |     hexagon: | TODO |
+    |        ia64: | TODO |
+    |        m68k: | TODO |
+    |  microblaze: | TODO |
+    |        mips: | TODO |
+    |       nds32: | TODO |
+    |       nios2: | TODO |
+    |    openrisc: | TODO |
+    |      parisc: | TODO |
+    |     powerpc: |  ok  |
+    |       riscv: | TODO |
+    |        s390: |  ok  |
+    |          sh: |  ok  |
+    |       sparc: |  ok  |
+    |          um: | TODO |
+    |   unicore32: | TODO |
+    |         x86: |  ok  |
+    |      xtensa: | TODO |
+    -----------------------