drivers: flash: fix memcpy and invalidate dcache in hyperflash

Replace memcpy usage in write since it is not linked to non-external
flash which might cause read-while-write issues.
Move the function that invalidated the data cache inside the critical
section to avoid context switch before it is invalidated.

Signed-off-by: Jeppe Odgaard <jeppe.odgaard@prevas.dk>