MIPS: ralink: Select CONFIG_CPU_MIPSR2_IRQ_VI on MT7620/8

Testing has shown, that when using mainline U-Boot on MT7688 based
boards, the system may hang or crash while mounting the root-fs. The
main issue here is that mainline U-Boot configures EBase to a value
near the end of system memory. And with CONFIG_CPU_MIPSR2_IRQ_VI
disabled, trap_init() will not allocate a new area to place the
exception handler. The original value will be used and the handler
will be copied to this location, which might already be used by some
userspace application.

The MT7688 supports VI - its config3 register is 0x00002420, so VInt
(Bit 5) is set. But without setting CONFIG_CPU_MIPSR2_IRQ_VI this
bit will not be evaluated to result in "cpu_has_vi" being set. This
patch now selects CONFIG_CPU_MIPSR2_IRQ_VI on MT7620/8 which results
trap_init() to allocate some memory for the exception handler.

Please note that this issue was not seen with the Mediatek U-Boot
version, as it does not touch EBase (stays at default of 0x8000.0000).
This is strictly also not correct as the kernel (_text) resides
here.

Signed-off-by: Stefan Roese <sr@denx.de>
[paul.burton@mips.com: s/beeing/being/]
Signed-off-by: Paul Burton <paul.burton@mips.com>
Cc: John Crispin <blogic@openwrt.org>
Cc: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: linux-mips@linux-mips.org