of/irq: Rename "intc_desc" to "of_intc_desc" to fix OF on sh

Now CONFIG_OF can be enabled on sh:

    drivers/of/irq.c:472:8: error: redefinition of 'struct intc_desc'
    include/linux/sh_intc.h:109:8: note: originally defined here

As "intc_desc" is used all over the place in sh platform code, while
drivers/of/irq.c has a local definition used in a single function,
rename the latter by prefixing it with "of_".

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Grant Likely <grant.likely@linaro.org>