arm64: imx: Deduplicate DH i.MX6 DHSOM defconfig

arm64: imx: Deduplicate DH i.MX6 DHSOM defconfig

Deduplicate defconfigs for all DH i.MX6 DHSOM by including common
configs/imx_dhsom_defconfig . This does introduce changes to the
board configuration, namely it enables commands used on all DHSOM
devices consistenty, the prompt is changed to u-boot=>, support for
booting non-Linux OS which was likely never used is disabled, GPT
partition table support is enabled, generic MTD support is enabled,
LED support is enabled, DM PHY, PMIC and regulator support is also
enabled, KASLR command is enabled.

Signed-off-by: Marek Vasut <marex@denx.de>