axs103: Bump CPU frequency from 50MHz to 100MHz

In the upcoming release of axs103 v1.1 CPU will
run @100MHz which we support with that change.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>

configs/axs103_defconfig

 CONFIG_ARC=y
 CONFIG_ISA_ARCV2=y
 CONFIG_DM_SERIAL=y
-CONFIG_SYS_CLK_FREQ=50000000
+CONFIG_SYS_CLK_FREQ=100000000
 CONFIG_SYS_TEXT_BASE=0x81000000
 CONFIG_DEFAULT_DEVICE_TREE="axs10x"
 CONFIG_BOOTDELAY=3