toolchain/toolchain-external/toolchain-external-arm-arm: add dependency on NEON

While testing Buildroot on a Cortex-A5 that doesn't provide NEON, we
found out that a system generated with the ARM toolchain from Arm
didn't boot. It turns out that this ARM toolchain is built with:

  --with-arch=armv7-a --with-fpu=neon --with-float=hard --with-mode=thumb

So, it uses NEON as its FPU, which means it can only work on CPU cores
that have NEON support. This commit adds the appropriate dependency to
the toolchain-external-arm-arm package, and adjusts the Config.in help
text accordingly.

While at it, it also drops the part of the Config.in help text that
says the code is tuned for Cortex-A9, as it is not the case: it was
the case for the Linaro toolchain (built with --with-tune=cortex-a9),
but not for the ARM toolchain, for which no specific --with-tune is
passed.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Cc: Alexandre Belloni <alexandre.belloni@bootlin.com>
Cc: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>