Convert CONFIG_SPI to Kconfig

This converts the following to Kconfig:
   CONFIG_SPI

This partly involves updating code that assumes that CONFIG_SPI implies
things that are specific to the MPC8xx SPI driver.  For now, just update
the CONFIG tests.  This also involves reworking the default for
CONFIG_SYS_DEF_EEPROM_ADDR so that we don't set it when we cannot make a
reasonable default, as it does not cause any compile failures.

Signed-off-by: Adam Ford <aford173@gmail.com>
Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/netgear_cg3100d_ram_defconfig b/configs/netgear_cg3100d_ram_defconfig
index f1ceb5c..90cd69e 100644
--- a/configs/netgear_cg3100d_ram_defconfig
+++ b/configs/netgear_cg3100d_ram_defconfig
@@ -45,6 +45,7 @@
 # CONFIG_SPL_SERIAL_PRESENT is not set
 CONFIG_DM_SERIAL=y
 CONFIG_BCM6345_SERIAL=y
+CONFIG_SPI=y
 CONFIG_DM_SPI=y
 CONFIG_BCM63XX_SPI=y
 CONFIG_WDT_BCM6345=y