ARM: samsung: fix PM debug build with DEBUG_LL but !MMU

Selecting CONFIG_SAMSUNG_PM_DEBUG (depending on CONFIG_DEBUG_LL) but
without CONFIG_MMU leads to build errors:

  arch/arm/plat-samsung/pm-debug.c: In function ‘s3c_pm_uart_base’:
  arch/arm/plat-samsung/pm-debug.c:57:2: error:
    implicit declaration of function ‘debug_ll_addr’ [-Werror=implicit-function-declaration]

Fixes: 99b2fc2b ("ARM: SAMSUNG: Use debug_ll_addr() to get UART base address")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Cc: <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20200910154150.3318-1-krzk@kernel.org