dfu: dfu_sf: Take the start address into account

dfu: dfu_sf: Take the start address into account

The dfu_alt_info_spl variable allows passing a starting point
for the binary to be flashed in the SPI NOR.

For example, if we have 'dfu_alt_info_spl=spl raw 0x400', this means
that we want to flash the binary starting at address 0x400.

In order to do so we need to erase the entire sector and write to
the the subsequent SPI NOR sectors taking such start address
into account for the address calculations.

Tested by succesfully writing SPL binary into 0x400 offset and
the u-boot.img at offset 64 kiB of a SPL NOR.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Lukasz Majewski <l.majewski@samsung.com>
[trini: Use lldiv for the math]
Signed-off-by: Tom Rini <trini@konsulko.com>