mtd: rawnand: meson: refine resource getting in probe

simply use devm_platform_ioremap_resource_byname() instead of two steps:
res = platform_get_resource(pdev, IORESOURCE_MEM, 0) and
reg_base = devm_ioremap_resource(dev, res)

Reviewed-by: Kevin Hilman <khilman@baylibre.com>
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Liang Yang <liang.yang@amlogic.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20220907080405.28240-4-liang.yang@amlogic.com