core: drivers: support SiFive UART

Add sifive uart support.

Signed-off-by: Yu-Chien Peter Lin <peter.lin@sifive.com>
Reviewed-by: Samuel Holland <samuel.holland@sifive.com>
Reviewed-by: Zong Li <zo

core: drivers: support SiFive UART

Add sifive uart support.

Signed-off-by: Yu-Chien Peter Lin <peter.lin@sifive.com>
Reviewed-by: Samuel Holland <samuel.holland@sifive.com>
Reviewed-by: Zong Li <zong.li@sifive.com>
Acked-by: Alvin Chang <alvinga@andestech.com>
Acked-by: Etienne Carriere <etienne.carriere@foss.st.com>

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