Plat AXG: Fix PLAT_MAX_PWR_LVL value

Plat AXG: Fix PLAT_MAX_PWR_LVL value

This patch fixes AXG platform build error:
plat/amlogic/axg/axg_pm.c: In function 'axg_pwr_domain_off':
plat/amlogic/axg/axg_pm.c:124:43: error: array subscript 2
 is above array bounds of 'const plat_local_state_t[2]'
 {aka 'const unsigned char[2]'}
by changing PLAT_MAX_PWR_LVL from MPIDR_AFFLVL1 to MPIDR_AFFLVL2
in plat\amlogic\axg\include\platform_def.h.

Change-Id: I9a701e8f26231e62f844920aec5830664f3fb324
Signed-off-by: Alexei Fedorov <Alexei.Fedorov@arm.com>