sh: remove unnecessary memset after alloc_bootmem_low_pages

Because alloc_bootmem functions return the allocated memory always
zeroed, an additional call of memset on allocated memory is
unnecessary.

Signed-off-by: Marek Skuczynski <M.Skuczynski@adbglobal.com>
Signed-off-by: Carl Shaw <carl.shaw@st.com>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>