ARM: OMAP2+: Fix imprecise external abort caused by bogus SRAM init

Some omaps are producing imprecise external aborts because we are
wrongly trying to init SRAM for device tree based booting. Only
omap3 is still using the legacy SRAM code, so we need to make it
omap3 specific. Otherwise we can get errors like this on at least
dm814x:

Unhandled fault: imprecise external abort (0xc06) at 0xc08b156c
...
(omap_rev) from [<c08b12e0>] (omap_sram_init+0xf8/0x3e0)
(omap_sram_init) from [<c08aca0c>] (omap_sdrc_init+0x10/0xb0)
(omap_sdrc_init) from [<c08b581c>] (pdata_quirks_init+0x18/0x44)
(pdata_quirks_init) from [<c08b5478>] (omap_generic_init+0x10/0x1c)
(omap_generic_init) from [<c08a57e0>] (customize_machine+0x1c/0x40)
(customize_machine) from [<c00098a4>] (do_one_initcall+0x80/0x1dc)
(do_one_initcall) from [<c08a2ec4>] (kernel_init_freeable+0x218/0x2e8)
(kernel_init_freeable) from [<c063a554>] (kernel_init+0x8/0xec)
(kernel_init) from [<c000f890>] (ret_from_fork+0x14/0x24)

Let's fix the issue by making sure omap_sdrc_init only gets called for
omap3. To do that, we need to have compatible "ti,omap3" in the dts
files. And let's also use "ti,omap3630" instead of "ti,omap36xx" like
we're supposed to.

Signed-off-by: Tony Lindgren <tony@atomide.com>