arm: conditionally support regular ARM instructions

Until now, all ARM processors supported the original ARM instructions.
However, the Cortex-M variants don't support them, and support only
Thumb/Thumb2 modes.

So, make a Kconfig option for ARM support and use it.

[Thomas:
  - Remove the dependency in the choice between ARM/Thumb/Thumb-2,
    because basically the choice is now always visible.
  - Replace the BR2_ARM_INSTRUCTIONS_ARM_CHOICE choice option directly
    by BR2_ARM_INSTRUCTIONS_ARM, instead of having this blind option
    defined separately. This means the choice is now always visible,
    even when only the ARM instruction set is supported.]

Signed-off-by: Guido Martínez <guido@vanguardiasur.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>