Beyond the CH_C_QOS register, two other registers whose offset is
related to channel number have fields within them.

Define the fields within the CH_C_CNTXT_0 GSI register, using an
enumerated type to identify the register's fields, and define an
array of field masks to use for that register's reg structure.

For the CH_C_CNTXT_1 GSI register, ch_c_cntxt_1_length_encode()
previously hid the difference in bit width in the channel ring
length field.  Instead, define a new field CH_R_LENGTH and encode
the ring size with reg_encode().
Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>