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

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 <[email protected]>
Cc: Vineet Gupta <[email protected]>
Cc: Noam Camus <[email protected]>
Cc: Tom Rini <[email protected]>