mfd: syscon: Remove "base" field from private driver data

During a move to use managed resources, the .remove function all was
deemed superfluous which subsequently led to its deletion. As there are
no remaining users of the stored 'base' address we only have to use it
locally during probe.

Signed-off-by: Alexander Shiyan <shc_work@mail.ru>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>