ARC: HSDK: Fix CPU frequency value

CPU on HSDK board runs at 500MHz after preloader so fix
wrong CPU frequency value in hsdk_defconfig.
Signed-off-by: NEugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>
Signed-off-by: NAlexey Brodkin <abrodkin@synopsys.com>

configs/hsdk_defconfig

@@ -2,7 +2,7 @@ CONFIG_ARC=y
 CONFIG_ISA_ARCV2=y
 CONFIG_TARGET_HSDK=y
 CONFIG_SYS_TEXT_BASE=0x81000000
-CONFIG_SYS_CLK_FREQ=1000000000
+CONFIG_SYS_CLK_FREQ=500000000
 CONFIG_DEFAULT_DEVICE_TREE="hsdk"
 CONFIG_USE_BOOTARGS=y
 CONFIG_BOOTARGS="console=ttyS0,115200n8"