arm64: interrupts: print FAR_ELx on sync exceptions

Default synchronous exceptions handler prints only esr and register
dump. Sometimes it requiers to see an address which caused exceptions
to understand what's going on

ARM ARM in section D13.2.41 states that FAR_EL2 will contain meanfull
value in case of ESR.EC holds 0x20, 0x21, 0x24, 0x25, 0x22, 0x34 or
0x35. Same applies for EL1.

This patch adds function whivh determine current EL, gets correct FAR
register and prints it on panic.

Signed-off-by: Pavel Skripkin <paskripkin@gmail.com>