xtensa: userspace: use ZSR_RFI_LEVEL for userspace enter

In xtensa_userspace_enter, we were hard-coding to use level 2
interrupt return mechanism to pivot to user mode and start
running the user thread. However, EPC2 and EPS2 may be used
for other purposes, and they could be used for interrupt
return if there are only two level interrupts. So change
the userspace enter to use ZSR_RFI_LEVEL, ZSR_EPC and ZSR_EPS
instead to be more explicit.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>