hw/timer/imx_*: fix TIMER_MAX clash with system symbol

The symbol TIMER_MAX used in imx_epit.c and imx_gpt.c
clashes with system symbol with the same name.  Because
all qemu source files includes qemu-common.h which, in
turn, includes limits.h, which is not unusual to define
it.  Rename local symbol to have a reasonable prefix.

Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
