arch/xtensa: Use ZSR assignments for interrupt return
arch/xtensa: Use ZSR assignments for interrupt return

We had a similar sequence for interrupt return, where we were
selecting (actually only for the benefit of qemu) the highest priority
EPCn/EPSn registers for our RFI instruction.  That works much better
in python the preprocessor.

Signed-off-by: Andy Ross <c70f9a6bf6ee0cd69c8af4ce5e6b131945769315@intel.com>

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