arm64: dts: renesas: beacon: Fix EEPROM compatible value

"make dtbs_check" fails with:

    arch/arm64/boot/dts/renesas/r8a774b1-beacon-rzg2n-kit.dt.yaml: eeprom@50: compatible: 'oneOf' conditional failed, one must be fixed:
	    'microchip,at24c64' does not match '^(atmel|catalyst|microchip|nxp|ramtron|renesas|rohm|st),(24(c|cs|lc|mac)[0-9]+|spd)$'

Fix this by dropping the bogus "at" prefix.

Fixes: a1d8a344 ("arm64: dts: renesas: Introduce r8a774a1-beacon-rzg2m-kit")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20210128110136.2293490-1-geert+renesas@glider.be