lib: Makefile: build crc7.c when CONFIG_MMC_SPI

When build U-Boot SPL, meet an issue of undefined reference to
'crc7' for drivers/mmc/mmc_spi.c, so let's compile crc7.c when
CONFIG_MMC_SPI selected.

Signed-off-by: Pragnesh Patel <pragnesh.patel@sifive.com>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Acked-by: Jagan Teki <jagan@amarulasolutions.com>
diff --git a/lib/Makefile b/lib/Makefile
index 6e688af..dc57619 100644
--- a/lib/Makefile
+++ b/lib/Makefile
@@ -30,7 +30,6 @@
 endif
 endif
 obj-$(CONFIG_USB_TTY) += circbuf.o
-obj-y += crc7.o
 obj-y += crc8.o
 obj-y += crc16.o
 obj-$(CONFIG_ERRNO_STR) += errno_str.o
@@ -90,6 +89,7 @@
 obj-y += display_options.o
 CFLAGS_display_options.o := $(if $(BUILD_TAG),-DBUILD_TAG='"$(BUILD_TAG)"')
 obj-$(CONFIG_BCH) += bch.o
+obj-$(CONFIG_MMC_SPI) += crc7.o
 obj-y += crc32.o
 obj-$(CONFIG_CRC32C) += crc32c.o
 obj-y += ctype.o