u-boot: Add kevin chromebook configs

This adds U-Boot configuration for the Samsung Chromebook Plus (v1),
also known as "chromebook_kevin" in the U-Boot upstream defconfigs. Also
adds a shared "gru" board directory to share with others having the same
baseboard.

It uses v2022.07 with some quality-of-life patches. The first one is a
clock adjustment to match coreboot clocks for the video output, the
second one is a series about text cursor support and larger fonts. These
are because the display has a high resolution of 2400x1600 at 12.3".

The config has the following diffconfig from the upstream defconfig for
this board:

    # For chainloading from depthcharge like a payload (RW_LEGACY).
    # Not everything might be necessary, but didn't test without these.
     INIT_SP_RELATIVE n -> y
     LNX_KRNL_IMG_TEXT_OFFSET_BASE 0x00200000 -> 0x18000000
     POSITION_INDEPENDENT n -> y
     SYS_TEXT_BASE 0x00200000 -> 0x18000000
    +SYS_INIT_SP_BSS_OFFSET 524288

    # Higher speeds for eMMC
     MMC_HS200_SUPPORT n -> y
     MMC_HS400_ES_SUPPORT n -> y
     MMC_HS400_SUPPORT n -> y
     MMC_IO_VOLTAGE n -> y
     MMC_SDHCI_SDMA n -> y
     MMC_SPEED_MODE_SET n -> y
    +MMC_UHS_SUPPORT y

    # Build the u-boot.elf to use as a payload
     REMAKE_ELF n -> y

    # Slightly faster video output
     VIDEO_COPY n -> y

    # Larger fonts per the applied series
     VIDEO_FONT_8X16 y -> n
     VIDEO_FONT_TER16X32 n -> y

Signed-off-by: Alper Nebi Yasak <alpernebiyasak@gmail.com>