ARM: exynos: Fix build error if SERIAL is disabled in SPL

ARM: exynos: Fix build error if SERIAL is disabled in SPL

If CONFIG_SPL_SERIAL_SUPPORT is not defined in include/configs/exynos5-common.h
the following error is produced during the build of the SPL:

arch/arm/mach-exynos/built-in.o: In function `do_lowlevel_init':
...u-boot/arch/arm/mach-exynos/lowlevel_init.c:221: undefined reference to `debug_uart_init'

Add additional condition to check if SPL build is in progress and
in that case check if CONFIG_SPL_SERIAL_SUPPORT is also set before
enabling the debug UART.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Simon Glass <sjg@chromium.org>
Cc: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>