mfd: syscon: Remove repetition of the regmap_get_val_endian()

Since the commit 0dbdb76c ("regmap: mmio: Parse endianness
definitions from DT") regmap MMIO parses DT itsef, no need to
repeat this in the caller(s).

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Lee Jones <lee@kernel.org>
Link: https://lore.kernel.org/r/20220808140811.26734-1-andriy.shevchenko@linux.intel.com