sh: Provide a 29/32-bit physical hint for bootloaders.

Shoves a magic word in to the empty_zero_page section for the
bootloader to work out whether to start the kernel in 29-bit
or 32-bit mode.

[ Renesas CPUs already take care of the initial PMB mappings entirely
  in hardware and decide on 29-bit/32-bit physical depending on which
  pin powered up the CPU, so this is mostly for ST parts. -- PFM ].

Signed-off-by: Stuart Menefy <stuart.menefy@st.com>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>