armv7m: Disable D-cache when booting nommu(ARMv7M) Linux kernel

Disable D-Cache is required when booting nommu Linux kernel.
(please see Linux kernel source "arch/arm/kernel/head-nommu.S")

U-Boot is enabled D-cache and I-Cache at startup.
However, it does not disable D-Cache before
booting nommu Linux kernel.
Therefore, I call dcache_disable()
when the CPU is ARMv7M to fix this problem.

Signed-off-by: Toshifumi NISHINAGA <tnishinaga.dev@gmail.com>