drivers: flash: w25qxxdv: write across programming pages

Until now spi_flash_wb_write/read functions were only supporting
buffers that fit into a W25Q page (even if there were not checking
the offset was page aligned).
This change allows these function to read/write multiple W25Q page.
This change is required as the caller of these functions are not
aware of flash page size.

Signed-off-by: Johannes Hutter <johannes@proglove.de>
Signed-off-by: Olivier Martin <olivier.martin@proglove.de>
Signed-off-by: Michael Wendland <michael@proglove.de>