mailbox: Remove depends on COMPILE_TEST for BCM_FLEXRM_MBOX

The Broadcom FlexRM mailbox driver uses platform MSI support but
not all ARCHs provide asm/msi.h. Due to this, we get compilation
error in Broadcom FlexRM mailbox driver via linux/msi.h on ARCHs
which lack asm/msi.h.

This patch removes "depends on COMPILE_TEST" for Kconfig option
BCM_FLEXRM_MBOX so that Broadcom FlexRM mailbox driver is only
compiled for ARM64.

Signed-off-by: Anup Patel <anup.patel@broadcom.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>