ARC: use fixed frequencies in arc_set_early_base_baud()

UARTs usually have fixed clock so we're switching to use of
constant values instead of something derived from core clock
frequency.

Among other things this will allow us to get rid of
arc_{get|set}_core_freq() and switch to generic clock
framework later on.

Acked-by: Christian Ruppert <christian.ruppert@alitech.com>
Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>