fastboot: rockchip: Select FASTBOOT_FLASH[_MMC_DEV]

Select CONFIG_FASTBOOT_FLASH, CONFIG_FASTBOOT_FLASH_MMC_DEV
for rockchip SoC plaforms in fastboot Kconfig file instead
of defined it in board defconfig.

This eventually drop the explicit configs defined in
supporting board defconfig files.

Tested-by: Levin Du <djw@t-chip.com.cn>
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>