mfd: Fix compile for twl-core.c by removing cpu_is_omap usage

Commit 7d7e1eba (ARM: OMAP2+: Prepare for irqs.h removal) broke
compile for non-omap as include plat/cpu.h was added. This header
was indirectly included earlier when SPARSE_IRQ was not set, but
does not exist on most platforms.

Fix the problem by removing the cpu_is_omap usage that should
not exist in drivers at all. We can do this by adding proper
clock aliases for the twl-core.c drivers, and drop separate
handling for cases when clock framework is not available as
the behaviour will stay the same.

Note that we need to add a platform device to avoid using the
i2c provided names that may be different on various omaps.

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: Paul Walmsley <paul@pwsan.com>
Acked-by: Samuel Ortiz <sameo@linux.intel.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Cc: Olof Johansson <olof@lixom.net>
Cc: Felipe Balbi <balbi@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>