LoongArch: Disable FIX_EARLYCON_MEM when ARCH_IOREMAP is enabled

When ARCH_IOREMAP is enabled, we are using always accessible DMW for
ioremap(). It makes no sense to create a dedicated mapping for earlycon
given that we can access the region via DMW.

Disable FIX_EARLYCON_MEM when ARCH_IOREMAP is selected. This can ease
debugging for early mapping issues.

Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>