rtc: rtc-rs5c372: SMBus conversion/support

rtc-rs5c372 presently depends on I2C master mode transfers, despite the
fact that these RTCs frequently find themselves on SMBus-only adapters.

Given that the only capabilities that were checked were for I2C_FUNC_I2C,
it's assumed that most of the adapters that are currently using this
driver are fairly sane, and are able to handle SMBus emulation (though we
adjust the default capabilities to check for I2C_FUNC_SMBUS_EMUL anyways,
which is the vast majority of them.  The adapters that don't have their
own ->smbus_xfer() fall back on the ->master_xfer() through the emulated
transfer).

The special case is iop3xx, which has more than its fair share of hacks
within this driver, it remains untested -- though also claims to support
emulated SMBus accesses.  The corner case there is rs5c_get_regs() which
uses access mode #3 for transferring the register state, while we use mode
#1 for SMBus.
Signed-off-by: Paul Mundt <lethal@linux-sh.org>
Acked-by: David Brownell <david-b@pacbell.net>
Tested-by: Riku Voipio <riku.voipio@movial.fi>
Acked-by: Alessandro Zummo <a.zummo@towertech.it>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>