CONFIG_STACKSIZE is not referenced anywhere except on AVR32, but present
in most ARM board config files.

IRQs are only enabled for 1 config, so remove the unused config options
for IRQ and FIQ stack size as well.

Cc: Albert ARIBAUD <albert.u.boot@aribaud.net>
Signed-off-by: Rob Herring <rob.herring@calxeda.com>