romlayout: Use "rep ; nop" not "rep nop".

Fixes:
  Compiling (16bit) out/romlayout.o
src/romlayout.S: Assembler messages:
src/romlayout.S:285: Error: expecting string instruction after `rep'
make: *** [out/romlayout.o] Error 1

Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Ian Campbell <ian.campbell@citrix.com>