mfd: syscon: atmel-smc: Add new helpers to ease SMC regs manipulation

These new helpers + macro definitions are meant to replace the old ones
which are unpractical to use.

Note that the macros and function prefixes have been intentionally
changed to ATMEL_[H]SMC_XX and atmel_[h]smc_ to reflect the fact that
this IP is also embedded in avr32 SoCs (and not only in at91 ones).

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Acked-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>