doc: Fix statements on case sensitivity for regulator prefix

The regulator bindings state that regulator prefixes are allowd to be
in upper or lower case. However pmic_bind_children from pmic_uclass uses
strncmp to compare DT node name against prefix. This comparison is case
sensitive hence the regulator driver prefix case matters.

Signed-off-by: Felix Brack <fb@ltec.ch>

doc/device-tree-bindings/regulator/regulator.txt

@@ -10,10 +10,10 @@ pmic: drivers/power/pmic/max77686.c
 regulator: drivers/power/regulator/max77686.c
 
 For the node name e.g.: "prefix[:alpha:]num { ... }":
-- the driver prefix should be: "prefix" or "PREFIX" - case insensitive
+- the driver prefix should be: "prefix" - case sensitive
 - the node name's "num" is set as "dev->driver_data" on bind
 
-Example the prefix "ldo" will pass for: "ldo1", "ldo@1", "LDO1", "LDOREG@1"...
+Example the prefix "ldo" will pass for: "ldo1", "ldo@1", "ldoreg@1, ...
 
 Optional properties:
 - regulator-name: a string, required by the regulator uclass