mtd: rawnand: meson: use of_property_count_elems_of_size helper

Use the of_property_count_elems_of_size() helper instead of open-coding
it's logic. As a bonus this will now error out if the "reg" property
values use an incorrect size (anything other than sizeof(u32)).

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Tested-by: Liang Yang <liang.yang@amlogic.com>
Acked-by: Liang Yang <liang.yang@amlogic.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>