mtd: rawnand: denali_dt: make the core clock optional

The "nand_x" and "ecc" clocks are currently optional. Make the core
clock optional in the same way. This will allow platforms with no clock
driver support to use this driver.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Tested-by: Marek Vasut <marex@denx.de> # On SoCFPGA Arria V