xtensa: make CONFIG_XTENSA_MMU_PTEVADDR based on choice

Instead of being able to arbitrarily set the PTEVADDR for page
table, this provides choices (currently just one). This is in
preparation to enable handling memory management exception in
C code. For that to work, we will need to pre-load the page
table address (PTEVADDR) for the memory page containing
exception code and data (containing jump addresses), and
various stacks. This is to prempt any TLB misses during handling
the level 1 interrupt code. If a TLB miss is encountered during
handling of level 1 interrupt, we will be thrown into double
exception handling code where we will get stuck in infinite
loop. However, in order to pre-load the page table entries,
PTEVADDR needs to be calculated. This requires the use of
PTEVADDR base which cannot be loaded via l32r, as we may cause
a data TLB miss. So we must be able to grab the PTEVADDR base
address strictly within code, and must be without any data
load. So changing CONFIG_XTENSA_MMU_PTEVADDR to be based on
choice so we can have pre-defined bit shift value for shift
operation. This shift value will be used in exception handling
code.

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