mfd: Remove unused helper syscon_regmap_lookup_by_pdevname

Nobody uses the exported helper syscon_regmap_lookup_by_pdevname,
to lookup a device by name. Let us remove it.

Suggested-by: Arnd Bergman <arnd@arnd.de>
Cc: Arnd Bergman <arnd@arnd.de>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: "Rafael J. Wysocki" <rafael@kernel.org>
Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>