arm: apple: Don't clear framebuffer

Enable CONFIG_NO_FB_CLEAR to preserve the Asahi logo. Since that
logo is drawn on a black background also enable
CONFIG_SYS_WHITE_ON_BLACK such that text printed by U-Boot is still
visible.

Signed-off-by: Mark Kettenis <kettenis@openbsd.org>

configs/apple_m1_defconfig

@@ -13,5 +13,7 @@ CONFIG_NVME_APPLE=y
 CONFIG_USB_XHCI_HCD=y
 CONFIG_USB_XHCI_DWC3=y
 CONFIG_USB_KEYBOARD=y
+CONFIG_SYS_WHITE_ON_BLACK=y
+CONFIG_NO_FB_CLEAR=y
 CONFIG_VIDEO_SIMPLE=y
 # CONFIG_GENERATE_SMBIOS_TABLE is not set