samples: drivers: spi_flash: Use valid addresses/sizes for dual-flash

When a board contains two identical flash memories configured in
parallel dual-flash mode, the actual sector size is twice the sector
size of a single flash memory. So, assuming 4-KiB sectors for each flash
memory, any erase operation must be 8-KiB aligned.

This commit updates the start offset and sector size in the spi_flash
sample to be properly aligned when a dual-flash configuration is used.

At the moment, this is only supported for STM32-based boards and QSPI
flash memories.

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