xtensa: add XIP-aware MTD support

Add Kconfig symbol, asm/mtd-xip.h header and define related macros:
xip_cpu_idle, xip_irqpending, xip_currtime and xip_elapsed_since.
Append .xiptext to the vectors when separate vectors are configured
because otherwise they are appended to the data and data may not even be
executable in XIP configuration.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>