mailbox: mailbox-test: allow reserved areas in SRAM

When CONFIG_SRAM is enable and the SRAM region is found, the entire SRAM
region resource is requested and marked as occupied by SRAM driver even
if certain parts of regions is marked reserved.

It's quite possible that a small region of the SRAM is reserved for all
the mailbox communication and hence it may fail to request the region
as it's already marked busy region.

This patch tries to just do a ioremap of this mailbox memory region if
it finds it busy.

Cc: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>