MIPS: JZ4740: introduce CONFIG_MACH_INGENIC

In preparation for supporting Ingenic SoCs other than the JZ4740,
introduce MACH_INGENIC to Kconfig & move MACH_JZ4740 to a separate
entry selected by the board when appropriate. This allows MACH_INGENIC
to be used to enable things generic across Ingenic SoCs.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Cc: Andrew Bresticker <abrestic@chromium.org>
Patchwork: https://patchwork.linux-mips.org/patch/10130/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>