event/octeontx: fix build for gcc < 4.6

-Ofast option supported from gcc4.6 [1], for older versions using
"-O3 -ffast-math" instead.

[1] build error:
cc1: error: invalid option argument ‘-Ofast’

Also struct initialization build error [2] fixed.

[2] build error:
.../event/octeontx/ssovf_mbox.c: In function ‘mbox_send_request’:
.../event/octeontx/ssovf_mbox.c:95:9: error: unknown field ‘u64’
specified in initializer

Fixes: 32ff26393bb2 ("event/octeontx: add SSO HW device operations")

Signed-off-by: Ferruh Yigit <ferruh.yigit@intel.com>
Acked-by: Jerin Jacob <jerin.jacob@caviumnetworks.com>