csky: uaccess.h: Coding convention with asm generic

Using asm-generic/uaccess.h to prevent duplicated code:
 - Add user_addr_max which mentioned in generic uaccess.h
 - Remove custom definitions of KERNEL/USER_DS, get/set_fs,
   uaccess_kerenl
 - Using generic extable.h instead of custom definitions in
   uaccess.h

Change v2:
 - Fixup tinyconfig compile error, "__put_user_bad"
 - Add __get_user_asm_64

Signed-off-by: Guo Ren <guoren@linux.alibaba.com>
Link: https://lore.kernel.org/linux-csky/CAK8P3a1DvsXSEDoovLk11hzNHyJi7vqNoToU+n5aFi2viZO_Uw@mail.gmail.com/T/#mbcd58a0e3450e5598974116b607589afa16a3ab7
Cc: Arnd Bergmann <arnd@arndb.de>