x86: Select the ns16550 debug UART for minnowmax, chromebook_link

At present the debug UART is not selected which causes a build error.
Correct this.

Signed-off-by: Simon Glass <sjg@chromium.org>

configs/chromebook_link_defconfig

@@ -23,6 +23,7 @@ CONFIG_SPI_FLASH=y
 CONFIG_DM_PCI=y
 CONFIG_DM_RTC=y
 CONFIG_DEBUG_UART=y
+CONFIG_DEBUG_UART_NS16550=y
 CONFIG_DEBUG_UART_BASE=0x3f8
 CONFIG_DEBUG_UART_CLOCK=1843200
 CONFIG_DEBUG_UART_BOARD_INIT=y

configs/minnowmax_defconfig

@@ -23,6 +23,7 @@ CONFIG_DM_ETH=y
 CONFIG_DM_PCI=y
 CONFIG_DM_RTC=y
 CONFIG_DEBUG_UART=y
+CONFIG_DEBUG_UART_NS16550=y
 CONFIG_DEBUG_UART_BASE=0x3f8
 CONFIG_DEBUG_UART_CLOCK=1843200
 CONFIG_USB=y