serial: pl011: Enable DEBUG_UART_PL011 in SPL

Commit b81406db ("arm: serial: Add debug UART capability to the
pl01x driver") add supports to use pl01x as a debug UART. However,
due to CONFIG_IS_ENABLED macro requires CONFIG_SPL_* prefix, the
_debug_uart_init() would not choose TYPE_PL011 in SPL build. This
patch fixes the bug by judging CONFIG_DEBUG_UART_PL011 explicitly.

Signed-off-by: Chen Baozi <chenbaozi@phytium.com.cn>
Reviewed-by: Simon Glass <sjg@chromium.org>