serial/serial_arc: switch from {read|write}l to {read|write}b accessors

This is required for proper functionality on big-endian targets.
Memory-mapped registres of ARC UART are not 32-bit words but 8-bit bytes
so on little-endian target either acessor (_l or _b) works fine.
On big-endian only _b accessors works as expected.

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

Cc: Vineet Gupta <vgupta@synopsys.com>
Cc: Noam Camus <noamc@ezchip.com>
Cc: Tom Rini <trini@ti.com>