arch/xtensa: adsp: add SRAM window 2 initialization

Define SRAM window 2 details to the headers and open access to host to
the window in bootloader.

Also clarify Kconfig documentation that the semantics of the windows can
vary depending on which host<->DSP communication scheme is in use.

Signed-off-by: Kai Vehmanen <kai.vehmanen@linux.intel.com>