msm: ipa3: Added uC Event Ring memory region to IPA SRAM

Since there is no dedicated descriptor RAM for uC in SDX20 and
SDM845, the event ring is added to IPA SRAM. The transfer ring is
placed in uC DRAM, since IPA only needs read access to it. However
the event ring cannot be placed in uC DRAM, since IPA needs to write
to it and this is forbidden per security policy. uC event ring get 1K
space from the bottom of the IPA SRAM allocation on account of the
modem memory region.

Change-Id: I596528b9232824addb4722272edd478c45ba1f0b
Acked-by: Dmitry Kogan <dmitryk@qti.qualcomm.com>
Signed-off-by: Amir Levy <alevy@codeaurora.org>