xtensa: remove free_initrd_mem

The xtensa free_initrd_mem() verifies that initrd is mapped and then
frees its memory using free_reserved_area().

The initrd is considered mapped when its memory was successfully reserved
with mem_reserve().

Resetting initrd_start to 0 in case of mem_reserve() failure allows to
switch to generic free_initrd_mem() implementation.

Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>
Message-Id: <1563977432-8376-1-git-send-email-rppt@linux.ibm.com>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>