ARM: uniphier: remove workaround for the NAND write protect

This workaround was previously needed for LD4, Pro4, sLD8, Pro5
SoCs. The boot ROM does not touch this register for PXs2/LD6b or
later.

Now that the reset signal of the Denali NAND controller is always
asserted in board_init() then deasserted in the driver, the
WRITE_PROTECT register gets back to the default value, which means
the write protect is deasserted.

This workaround can go away entirely.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>