Fix kconfig dependency warnings and subsequent build errors:

WARNING: unmet direct dependencies detected for SERIAL_SUNPLUS
  Depends on [n]: TTY [=n] && HAS_IOMEM [=y] && (ARCH_SUNPLUS [=y] || COMPILE_TEST [=n])
  Selected by [y]:
  - SOC_SP7021 [=y] && ARCH_SUNPLUS [=y]

WARNING: unmet direct dependencies detected for SERIAL_SUNPLUS_CONSOLE
  Depends on [n]: TTY [=n] && HAS_IOMEM [=y] && SERIAL_SUNPLUS [=y]
  Selected by [y]:
  - SOC_SP7021 [=y] && ARCH_SUNPLUS [=y]

(samples, not all:)
drivers/tty/serial/sunplus-uart.c:342: undefined reference to `uart_get_baud_rate'
arm-linux-gnueabi-ld: drivers/tty/serial/sunplus-uart.c:379: undefined reference to `uart_update_timeout'
drivers/tty/serial/sunplus-uart.c:526: undefined reference to `uart_console_write'
arm-linux-gnueabi-ld: drivers/tty/serial/sunplus-uart.c:274: undefined reference to `tty_flip_buffer_push'
arm-linux-gnueabi-ld: drivers/tty/serial/sunplus-uart.o:(.data+0xa8): undefined reference to `uart_console_device'
drivers/tty/serial/sunplus-uart.c:720: undefined reference to `uart_register_driver'
arm-linux-gnueabi-ld: drivers/tty/serial/sunplus-uart.c:726: undefined reference to `uart_unregister_driver'
drivers/tty/serial/sunplus-uart.c:551: undefined reference to `uart_parse_options'
arm-linux-gnueabi-ld: drivers/tty/serial/sunplus-uart.c:553: undefined reference to `uart_set_options'

This is the same technique that is used 2 times in
arch/arm/mach-versatile/Kconfig.

Fixes: 0aa94eea ("ARM: sunplus: Add initial support for Sunplus SP7021 SoC")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Qin Jian <qinjian@cqplus1.com>
Cc: Necip Fazil Yildiran <fazilyildiran@gmail.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: linux-arm-kernel@lists.infradead.org
Cc: patches@armlinux.org.uk
Signed-off-by: Arnd Bergmann <arnd@arndb.de>