[ARM] 4187/1: iop: unify time implementation across iop32x, iop33x, and iop13xx

* architecture specific details are handled in asm/arch/time.h
* ARCH_IOP13XX now selects PLAT_IOP
* as suggested by Lennert use ifdef CONFIG_XSCALE to skip the cp_wait on
XSC3

Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>