MIPS: Use Ingenic-specific write combine attribute on all Ingenic platforms

MIPS: Use Ingenic-specific write combine attribute on all Ingenic platforms

The Ingenic-specific write combining cache attribute was defined based
on CONFIG_MACH_JZ4740 and therefore not used on JZ4780. Change this to
CONFIG_MACH_INGENIC so that it gets used on all Ingenic platforms.

Signed-off-by: Alex Smith <alex.smith@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/10769/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>