xtensa: Add Kconfig for asm2 layer

The asm2 layer will build alongside the traditional assembly, but the
reverse is not true.  Add a CONFIG_XTENSA_ASM2 to force its use at
runtime and disable the older code.

Note that the older assembly had an initialization function that is
properly part of the timer driver.  Move a C equivalent into the timer
driver itself for now to prevent a build breakage.  Long term we need
to clean that driver up in a bunch of other ways.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>