mfd: syscon: Add non-DT support

This patch allow using syscon driver from the platform data, i.e.
possibility using driver on systems without oftree support.
For search syscon device from the client drivers,
"syscon_regmap_lookup_by_pdevname" function was added.

Signed-off-by: Alexander Shiyan <shc_work@mail.ru>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>