soc: riscv: rv32m1: Fix optional context save
soc: riscv: rv32m1: Fix optional context save

Saving an extended context for RV32M1 should be optional, but it was
broken due to the offset calculation not taking the according option
into account.

Signed-off-by: Karsten Koenig <9871d5f9f33c7082866c34a25eef2f8270ae0185@gmail.com>

zephyrproject-rtos/zephyr,nashif/zephyr,nashif/zephyr,finikorg/zephyr,Vudentz/zephyr,finikorg/zephyr,nashif/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,finikorg/zephyr,galak/zephyr,nashif/zephyr,finikorg/zephyr,galak/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,Vudentz/zephyr,nashif/zephyr,Vudentz/zephyr,galak/zephyr