drivers: flash: stm32_qspi: Fix page size for dual-flash

When dual-flash mode is enabled, even bytes are written to the first
flash memory and odd bytes to the second flash memory. This means, from
the flash driver's point of view, the size of a flash page is twice the
size of a single flash memory's page.

So if each flash memory has 256-byte pages, 512 bytes should be used as
page size by the flash driver. Using 256 bytes was working fine but is
suboptimal.

Signed-off-by: Thomas Altenbach <altenbach.thomas@gmail.com>