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/sama5d3xek_mmc_defconfig b/configs/sama5d3xek_mmc_defconfig
index ea565ec..f114b2e 100644
--- a/configs/sama5d3xek_mmc_defconfig
+++ b/configs/sama5d3xek_mmc_defconfig
@@ -1,4 +1,5 @@
 CONFIG_ARM=y
+CONFIG_ARCH_CPU_INIT=y
 CONFIG_ARCH_AT91=y
 CONFIG_SYS_TEXT_BASE=0x26f00000
 CONFIG_TARGET_SAMA5D3XEK=y