soc: ite: ilm: it51xxx: Support RAM code size up to 4K

Previously, the RAM code size was limited to 1K, causing issues when the
code exceeded this limit. This update modifies the implementation to
support RAM code sizes up to 4K

test: zephyrproject/zephyr/tests/drivers/flash/common --> pass

Signed-off-by: Tim Lin <tim2.lin@ite.corp-partner.google.com>