flash/cfi_flash: Use virtual sector start address, not phys

include/flash.h was commented to say that the address in
flash_info->start was a physical address.  However, from u-boot's
point of view, and looking at most flash code, it makes more
sense for this to be a virtual address.  So I corrected the
comment to indicate that this was a virtual address.

The only flash driver that was actually treating the address
as physical was the mtd/cfi_flash driver.  However, this code
was using it inconsistently as it actually directly dereferenced
the "start" element, while it used map_physmem to get a
virtual address in other places.  I changed this driver so
that the code which initializes the info->start field calls
map_physmem to get a virtual address, eliminating the need for
further map_physmem calls.  The code is now consistent.

The *only* place a physical address should be used is when defining the
flash banks list that is used to initialize the flash_info struct,
usually found in the board config file.

Signed-off-by: Becky Bruce <beckyb@kernel.crashing.org>
Signed-off-by: Stefan Roese <sr@denx.de>