regulator: mc13xxx: Use lowercase regulator names to match the DT

Since c32569e3 ("regulator: Use of_node_name_eq for node name
comparisons") Vivien reported the mc13892-regulator complaining about
not being able to find regulators.

This is because prior to that commit we used of_node_cmp() to compare
the regulator array passed from mc13892_regulators down to
mc13xxx_parse_regulators_dt() and they are all defined in uppercase
letters by the MC13892_*_DEFINE* macros, whereas they are defined as
lowercase in the DTS.

Fix this by using a lowercase regulator name to match the DT node name.

Fixes: c32569e3 ("regulator: Use of_node_name_eq for node name comparisons")
Reported-by: Vivien Didelot <vivien.didelot@gmail.com>
Reported-by: Florian Fainelli <f.fainelli@gmail.com>
Cc: Liam Girdwood <lgirdwood@gmail.com>
Cc: Mark Brown <broonie@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>
Tested-by: Vivien Didelot <vivien.didelot@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>