drivers/flash/soc_flash_mcux: adjust alignment logic

The current alignment logic does not work as expected if given unaligned
values, resulting in a skip of the first word. The length also has to
take into account the starting address: for example, asking for 2 bytes
at offset 3 should actually check 8 bytes.

This patch adjusts the logic so that it always includes the first and
the last word of the input area.

Signed-off-by: Luca Burelli <l.burelli@arduino.cc>