xtensa: document boot parameter passing

Add booting.txt to Documentation/xtensa with description of boot
parameters representation and passing to the kernel.
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>

Documentation/xtensa/booting.txt

+Passing boot parameters to the kernel.
+
+Boot parameters are represented as a TLV list in the memory. Please see
+arch/xtensa/include/asm/bootparam.h for definition of the bp_tag structure and
+tag value constants. First entry in the list must have type BP_TAG_FIRST, last
+entry must have type BP_TAG_LAST. The address of the first list entry is
+passed to the kernel in the register a2. The address type depends on MMU type:
+- For configurations without MMU, with region protection or with MPU the
+  address must be the physical address.
+- For configurations with region translarion MMU or with MMUv3 and CONFIG_MMU=n
+  the address must be a valid address in the current mapping. The kernel will
+  not change the mapping on its own.
+- For configurations with MMUv2 the address must be a virtual address in the
+  default virtual mapping (0xd0000000..0xffffffff).
+- For configurations with MMUv3 and CONFIG_MMU=y the address may be either a
+  virtual or physical address. In either case it must be within the default
+  virtual mapping. It is considered physical if it is within the range of
+  physical addresses covered by the default KSEG mapping (XCHAL_KSEG_PADDR..
+  XCHAL_KSEG_PADDR + XCHAL_KSEG_SIZE), otherwise it is considered virtual.