Using get_current_el_maybe_constant() produces more optimized code
because in most cases, we know the exception level at build-time.
For example, BL31 runs at EL3, so unneeded code will be trimmed.

[before]

0000000000000000 <is_dcache_enabled>:
   0:   d5384240        mrs     x0, currentel
   4:   53020c00        ubfx    w0, w0, #2, #2
   8:   7100041f        cmp     w0, #0x1
   c:   54000081        b.ne    1c <is_dcache_enabled+0x1c>  // b.any
  10:   d5381000        mrs     x0, sctlr_el1
  14:   53020800        ubfx    w0, w0, #2, #1
  18:   d65f03c0        ret
  1c:   7100081f        cmp     w0, #0x2
  20:   54000061        b.ne    2c <is_dcache_enabled+0x2c>  // b.any
  24:   d53c1000        mrs     x0, sctlr_el2
  28:   17fffffb        b       14 <is_dcache_enabled+0x14>
  2c:   d53e1000        mrs     x0, sctlr_el3
  30:   17fffff9        b       14 <is_dcache_enabled+0x14>

[after]

0000000000000000 <is_dcache_enabled>:
   0:   d53e1000        mrs     x0, sctlr_el3
   4:   53020800        ubfx    w0, w0, #2, #1
   8:   d65f03c0        ret

Change-Id: I3698fae9b517022ff9fbfd4cad3a320c6e137e10
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>