x86/efi: Clean up a minor mistake in comment

EFI allocates runtime services regions from EFI_VA_START, -4G, down
to -68G, EFI_VA_END - 64G altogether, top-down.

The mechanism was introduced in commit:

  d2f7cbe7b2 ("x86/efi: Runtime services virtual mapping")

Fix the comment that still says bottom-up.

Signed-off-by: Baoquan He <bhe@redhat.com>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Matt Fleming <matt@codeblueprint.co.uk>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-efi@vger.kernel.org
Link: http://lkml.kernel.org/r/20170404160245.27812-10-ard.biesheuvel@linaro.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/platform/efi/efi_64.c

@@ -47,7 +47,7 @@
 #include <asm/pgalloc.h>
 
 /*
- * We allocate runtime services regions bottom-up, starting from -4G, i.e.
+ * We allocate runtime services regions top-down, starting from -4G, i.e.
  * 0xffff_ffff_0000_0000 and limit EFI VA mapping space to 64G.
  */
 static u64 efi_va = EFI_VA_START;