MIPS: Ingenic: Add Ingenic X1830 support.

Support the Ingenic X1830 SoC using the code under arch/mips/jz4740.
This is left unselectable in Kconfig until a X1830 based board is
added in a later commit.

Tested-by: 周正 (Zhou Zheng) <sernia.zhou@foxmail.com>
Signed-off-by: 周琰杰 (Zhou Yanjie) <zhouyanjie@wanyeetech.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>