ARM: rmobile: Convert CONFIG_R8A77xx to Kconfig

Convert these configuration options to Kconfig, update board defconfigs
and drop them from whitelist.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>
diff --git a/configs/porter_defconfig b/configs/porter_defconfig
index 7374a30..65ff3c0 100644
--- a/configs/porter_defconfig
+++ b/configs/porter_defconfig
@@ -1,6 +1,7 @@
 CONFIG_ARM=y
 CONFIG_ARCH_RMOBILE=y
 CONFIG_SYS_MALLOC_F_LEN=0x2000
+CONFIG_R8A7791=y
 CONFIG_TARGET_PORTER=y
 CONFIG_BOOTDELAY=3
 CONFIG_VERSION_VARIABLE=y