Define the GSI interrupt types with an enumerated type whose values
are the bit positions representing each interrupt type.  Include a
short comment describing how each interrupt type is used.

Build up the enabled interrupt mask explicitly in gsi_irq_enable(),
and get rid of the definition of GSI_CNTXT_TYPE_IRQ_MSK_ALL.
Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>