This value is unly used in arch/arm/mach-omap2/am33xx/
clock_am33xx.c, so let's make it dependent on AM33XX since
that is the only way this file gets compiled into the code
according to the Makefile.

Signed-off-by: Adam Ford <aford173@gmail.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
[trini: Fix symbol name]
Signed-off-by: Tom Rini <trini@konsulko.com>