Convert CONFIG_ARCH_CPU_INIT to Kconfig

This converts the following to Kconfig:
   CONFIG_ARCH_CPU_INIT

Signed-off-by: Adam Ford <aford173@gmail.com>
Acked-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Tested-by: Felix Brack <fb@ltec.ch>
diff --git a/configs/am335x_sl50_defconfig b/configs/am335x_sl50_defconfig
index b10d045..01fc52b 100644
--- a/configs/am335x_sl50_defconfig
+++ b/configs/am335x_sl50_defconfig
@@ -1,4 +1,5 @@
 CONFIG_ARM=y
+CONFIG_ARCH_CPU_INIT=y
 CONFIG_ARCH_OMAP2PLUS=y
 CONFIG_SPL_GPIO_SUPPORT=y
 CONFIG_SPL_LIBCOMMON_SUPPORT=y