dts: npcx: change the default memory configuration of npcx9m7fb

The internal flash size of npcx9m7fb is 512KB. Reduce the default
Code RAM size from 320KB to 256KB because the Code RAM size is limited
by FLASH_SIZE/2 in the Chromebook EC application.

Signed-off-by: Tom Chang <CHChang19@nuvoton.com>
Signed-off-by: Jun Lin <CHLin56@nuvoton.com>
(cherry picked from commit f2e11e42)