serial: sh: Fix compile error when lacking HSCIF support

If we attempt to compile serial_sh.c for a system which lacks HSCIF
support (e.g. R8A7740), we see the following compilation error:

    In file included from drivers/serial/serial_sh.c:20:
    drivers/serial/serial_sh.c: In function ‘sh_serial_init_generic’:
    drivers/serial/serial_sh.h:429:35: warning: implicit declaration of function ‘sci_HSSRR_out’; did you mean ‘sci_SCSCR_out’? [-Wimplicit-function-declaration]
      429 | #define sci_out(port, reg, value) sci_##reg##_out(port, value)
          |                                   ^~~~
    drivers/serial/serial_sh.c:62:17: note: in expansion of macro ‘sci_out’
       62 |                 sci_out(port, HSSRR, HSSRR_SRE | HSSRR_SRCYC8);
          |                 ^~~~~~~

To fix this, only try to support access to the HSSRR register for SoCs
where it actually exists.

Support for the RZ/G2L will be introduced in following patches, which
selects CONFIG_RCAR_64 but does not have HSCIF interfaces, so check for
CONFIG_RCAR_GEN2 || CONFIG_RCAR_GEN3 || CONFIG_RCAR_GEN4 to determine if
HSCIF is present.

Fixes: bbe36e29 ('serial: sh: Add HSCIF support for R-Car SoC')
Signed-off-by: Paul Barker <paul.barker.ct@bp.renesas.com>
Reviewed-by: Biju Das <biju.das.jz@bp.renesas.com>
Cc: Hai Pham <hai.pham.ud@renesas.com>
Cc: Simon Glass <sjg@chromium.org>
Reviewed-by: Marek Vasut <marek.vasut+renesas@mailbox.org>