The offsets for almost all GSI registers we use have different
offsets starting at IPA version 4.5.  Only two registers remain
in their original location.

In a way though, the new register locations are not *that*
different.  The entire group of affected registers has simply
been shifted down in memory by a fixed amount (0xd000).  So for
example, the channel context 0 register that has a base offset of
0x0001c000 for "older" hardware now has a base offset of 0x0000f000.

This patch aims to add support for IPA v4.5 registers at their new
offets in a way that minimizes the amount of code that needs to
change.  It is not ideal, but it avoids the need to maintain
a nearly complete set of additional register offset definitions.

The approach takes advantage of the fact that when accessing GSI
registers we do not access any of memory at lower end of the "gsi"
memory range (with two exceptions already noted).  In particular,
we do not access anything within the bottom 0xd000 bytes of the
GSI memory range.

For IPA version 4.5, after we map the GSI memory, we adjust the
virtual memory pointer downward by the fixed amount (0xd000).
That way, register accesses using the offsets defined by the
existing GSI_REG_*() macros will resolve to the proper locations
for IPA version 4.5.

The two registers *not* affected by this offset are accessed only
in gsi_irq_setup().  There, for IPA version 4.5, we undo the general
register adjustment by adding the fixed amount back to the virtual
address to access these registers.
Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>