[MTD] MAPS: Add parameter to amd76xrom to override rom window size

The 2 bits controlling the window size are often set to allow reading the
BIOS, but too small to allow writing, since the lock registers are 4MiB
lower in the address space than the data.  This is intended to prevent
flashing the bios, perhaps accidentally.

The bits are 6 and 7.  If both bits are set, it is a 5MiB window. If only
the 7 Bit is set, it is a 4MiB window.  Otherwise, it is a 64KiB window.

This parameter allows the driver to override the BIOS settings.

Signed-off-by: Ryan Jackson <rjackson@lnxi.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>