msm: peripheral-loader: Change IOMAP_SIZE from SZ_4M to SZ_1M

The ioremap call made for zeroing out trailing memory is currently
requesting for a 4M memory region. This is resulting in vmalloc
allocation failures on some platforms.
Instead, request for 1M memory locations and zero out the area
in smaller chunks to make this failure less probable.

CRs-Fixed: 613426

Change-Id: I554dc42cbb37ec893306bd1cc625230216b9b6c9
Signed-off-by: Deepak Katragadda <dkatraga@codeaurora.org>