sh: Fix an unusual memory initialisation error.

This fixes a problems with the set up of Linux memory:

 - When reserving memory at boot time, the code previously reserved
   the bottom page of memory, and then from one page up to the end of
   the bootmap. This had the desired effect, but was strictly speaking
   wrong, as the one page was actually whatever CONFIG_ZERO_PAGE_OFFSET
   had been set to.

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