configs: mt7622: use ARMv8 Generic Timer instead of mtk_timer

It's better to use the generic timer which is correctly initialized by
the ATF. The generic timer has higher resolution than the mtk_timer.

diff --git a/configs/mt7622_rfb_defconfig b/configs/mt7622_rfb_defconfig
index fa427d8..12f5291 100644
--- a/configs/mt7622_rfb_defconfig
+++ b/configs/mt7622_rfb_defconfig
@@ -51,8 +51,6 @@
 CONFIG_DM_SPI=y
 CONFIG_MTK_SNOR=y
 CONFIG_SYSRESET_WATCHDOG=y
-CONFIG_TIMER=y
-CONFIG_MTK_TIMER=y
 CONFIG_WDT_MTK=y
 CONFIG_LZO=y
 CONFIG_HEXDUMP=y