csky: drop asm/gpio.h wrapper

The asm/gpio.h header is gone now that all architectures just use
gpiolib, and so the redirect is no longer valid.
Acked-by: Guo Ren <guoren@kernel.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>

arch/csky/include/asm/Kbuild

 # SPDX-License-Identifier: GPL-2.0
 generic-y += asm-offsets.h
 generic-y += extable.h
-generic-y += gpio.h
 generic-y += kvm_para.h
 generic-y += mcs_spinlock.h
 generic-y += qrwlock.h