of/fdt: Implement use BIN_ATTR_SIMPLE macro for fdt sysfs attribute

The usage of the macro allows to remove the custom handler function,
saving some memory. Additionally the code is easier to read.

While at it also mark the attribute as __ro_after_init, as the only
modification happens in the __init phase.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Link: https://lore.kernel.org/r/20241122-sysfs-const-bin_attr-of-v1-1-7052f9dcd4be@weissschuh.net


Signed-off-by: Rob Herring (Arm) <robh@kernel.org>