drivers: flash_stm32_xspi: fix DT accessor for flash size

The flash size is the second part (size) of the first reg value, not the
first part (address) of a nonexistent second reg value.

Based-on-patch-by: Armin Brauns <armin.brauns@embedded-solutions.at>
Signed-off-by: Tim Pambor <tim.pambor@codewrights.de>