mtd: nand: denali_dt: add compatible strings for UniPhier SoC variants

Add two compatible strings for UniPhier SoC family.

"socionext,uniphier-denali-nand-v5a" is used on UniPhier sLD3, LD4,
Pro4, sLD8.

"socionext,uniphier-denali-nand-v5b" is used on UniPhier Pro5, PXs2,
LD6b, LD11, LD20.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>